<PAGE>
                                                                   EXHIBIT 10.28

August 20, 2001


Infonautics, Inc.
590 North Gulph Road
King of Prussia, PA  19406

Ladies and Gentlemen:

The undersigned has been advised that as of the date hereof the undersigned may
be deemed to be an "affiliate" of Tucows Inc., a Delaware corporation
("Tucows"), as the term "affiliate" is defined for purposes of paragraphs (c)
and (d) of Rule 145 of the Rules and Regulations (the "Rules and Regulations")
of the Securities and Exchange Commission (the "Commission") under the
Securities Act of 1933, as amended (the "Act"). Pursuant to the terms of the
Agreement and Plan of Merger, dated as of March 27, 2001 (the "Merger
Agreement"), among Infonautics, Inc. a Pennsylvania corporation ("Parent"), TAC
Merger Sub Corporation, a Delaware corporation and a direct wholly owned
subsidiary of Parent ("Sub"), and Tucows, at the Effective Time (as defined in
the Merger Agreement) Sub will be merged with and into Tucows (the "Merger"),
and Tucows will become a wholly owned subsidiary of Parent.

As a result of the Merger, the undersigned may receive shares of Common Stock,
no par value per share ("Parent Common Stock"), of Parent. The undersigned would
receive such shares in exchange for shares of capital stock of Tucows owned by
the undersigned.

The undersigned hereby represents and warrants to, and covenants with, Parent
that in the event the undersigned receives any Parent Common Stock in the
Merger:

          (A) The undersigned shall not make any sale, transfer or other
     disposition of Parent Common Stock in violation of the Act or the Rules and
     Regulations.

          (B) The undersigned has carefully read this letter and discussed its
     requirements and other applicable limitations upon the undersigned's
     ability to sell, transfer or otherwise dispose of Parent Common Stock, to
     the extent the undersigned has felt it necessary, with the undersigned's
     counsel or counsel for Tucows.

          (C) The undersigned has been advised that the issuance of shares of
     Parent Common Stock to the undersigned in the Merger will be registered
     under the Act by a Registration Statement on Form S-4. However, the
     undersigned has also been advised that because (i) at the time of the
     Merger's submission for a vote of the stockholders of Tucows the
     undersigned may be deemed an affiliate of Tucows and (ii) the distribution
     by the undersigned of Parent Common Stock has not been registered under the
     Act, the undersigned may not sell, transfer or otherwise dispose of Parent
     Common Stock issued to the undersigned in the Merger unless (a) such sale,
     transfer or other disposition has

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Infonautics, Inc.
August 20, 2001
Page 2


     been registered under the Act, (b) such sale, transfer or other disposition
     is made in conformity with the volume and other limitations imposed by Rule
     145 under the Act, or (c) in the opinion of counsel reasonably acceptable
     to Parent, such sale, transfer or other disposition is otherwise exempt
     from registration under the Act.

          (D) The undersigned understands that, except as may otherwise be
     contemplated by the terms of the Merger Agreement, Parent will be under no
     obligation to register the sale, transfer or other disposition of Parent
     Common Stock by the undersigned or on the undersigned's behalf under the
     Act or, except as set forth in section (G) below, to take any other action
     necessary in order to make compliance with an exemption from such
     registration available.

          (E) The undersigned understands that stop transfer instructions will
     be given to Parent's transfer agent with respect to Parent Common Stock
     owned by the undersigned and that there may be placed on the certificates
     for Parent Common Stock issued to the undersigned, or any substitutions
     therefor, a legend stating in substance:

          "The shares represented by this certificate were issued in a
          transaction to which Rule 145 under the Securities Act of 1933
          applies. The shares represented by this certificate may only be
          transferred in accordance with the terms of a letter agreement dated
          August 20, 2001, a copy of which agreement is on file at the principal
          offices of Tucows Inc."

          (F) The undersigned also understands that unless the transfer by the
     undersigned of the undersigned's Parent Common Stock has been registered
     under the Act or is a sale made in conformity with the provisions of Rule
     145 under the Act, Parent reserves the right, in its sole discretion, to
     place the following legend on the certificates issued to any transferee of
     shares from the undersigned:

          "The shares represented by this certificate were acquired from a
          person who received such shares in a transaction to which Rule 145
          under the Securities Act of 1933 applies. The shares have been
          acquired by the holder not with a view to, or for resale in connection
          with, any distribution thereof within the meaning of the Securities
          Act of 1933 and may not be offered, sold, pledged or otherwise
          transferred except in accordance with an exemption from the
          registration requirements of the Securities Act of 1933."

     It is understood and agreed that the legend set forth in paragraph E or F
     above shall be removed by delivery of substitute certificates without such
     legend if (i) the undersigned shall have delivered to Parent a copy of a
     letter from the staff of the Commission, or an opinion of counsel, in form
     and substance reasonably satisfactory to Parent, to the effect that such
     legend is not required for purposes of the Act, (ii) the undersigned shall
     have

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Infonautics, Inc.
August 20, 2001
Page 3

     delivered to Parent reasonably satisfactory evidence or representations
     that the shares represented by such certificates are being or have been
     transferred in a transaction made in conformity with the provisions of Rule
     145, (iii) one year shall have elapsed from the date the undersigned
     acquired the Parent Common Stock received in the Merger and the provisions
     of Rule 145(d)(2) are then available to the undersigned, or (iv) two years
     shall have elapsed from the date the undersigned acquired the Parent Common
     Stock received in the Merger and the provisions of Rule 145(d)(3) are then
     applicable to the undersigned.

          (G) It is understood and agreed that for so long as and to the extent
     necessary to permit the undersigned to sell the Parent Common Stock
     pursuant to Rule 145 and, to the extent applicable, Rule 144 under the Act,
     Parent shall (a) use its reasonable efforts to (i) file, on a timely basis,
     all reports and data required to be filed with the Commission by it
     pursuant to Section 13 of the Securities Exchange Act of 1934, and (ii) and
     furnish to the undersigned upon request a written statement as to whether
     or not Parent has complied with such reporting requirements during the 12
     months preceding any proposed sale of the Parent Common Stock by the
     undersigned pursuant to Rule 145, and (b) otherwise use its reasonable
     efforts to permit such sales pursuant to Rule 145 and Rule 144. Parent
     hereby represents to the undersigned that it has filed all reports required
     to be filed by it with the Commission under Section 13 of the Securities
     Exchange Act of 1934 during the preceding 12 months.

          (H) This letter may be executed in one or more counterparts, each of
     which shall be deemed an original, but all of which together shall
     constitute one and the same document.

          (I) If a court of competent jurisdiction determines that any provision
     of this letter is not enforceable or enforceable only if limited in time
     and/or scope, this letter shall continue in full force and effect with such
     provision stricken or so limited.

          (J) Counsel to and accountants for the parties hereto shall be
     entitled to rely upon this letter as needed.

          (K) This letter shall be enforceable by, and shall inure to the
     benefit of and be binding upon, the parties hereto and their respective
     successors and assigns. As used herein, the term "successors and assigns"
     shall mean, where the context so permits, heirs, executors, administrators,
     trustees and successor trustees, and personal and other representatives.


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Infonautics, Inc.
August 20, 2001
Page 4


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         -------------------------------------
                                         [Please print name of entity, if any]


                                         By:  /s/ Stanley Stern
                                             -----------------------------------

                                         Name: Stanley Stern
                                               ---------------------------------
                                                          (please print)

                                         Title:
                                                --------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
    -----------------------------------------

Name: Van Morris
      ---------------------------------------

Title: Chief Executive Officer and President
       --------------------------------------


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Infonautics, Inc.
August 20, 2001
Page 5

          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         Robert Young
                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By:  /s/ Robert Young
                                              ----------------------------------

                                         Name:
                                              ----------------------------------
                                                          (please print)

                                         Title:
                                               ---------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
   -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 6


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                       Very truly yours,


                                       For and on Behalf of Parman Holding Corp.
                                       -----------------------------------------
                                       [Please print name of entity, if any]


                                       By:  /s/ P. Veenendaal
                                            ------------------------------------

                                       Name: P.Veenendaal
                                             -----------------------------------
                                                         (please print)

                                       Title: Attorney in Fact
                                              ----------------------------------
                                                         (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
    -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 7


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         STI Ventures N.V.
                                         -------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ P.N. Blauw
                                             -----------------------------------

                                         Name: P.N. Blauw
                                               ---------------------------------
                                                          (please print)

                                         Title: Director
                                                --------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
    ----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------


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Infonautics, Inc.
August 20, 2001
Page 8


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         Hapoalim Nechasim (Menayot) Ltd.
                                         -----------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ Yacov Elinav     /s/ Zali Guter
                                             -----------------------------------

                                         Name: Yacov Elinav       Zali Guter
                                               ---------------------------------
                                                          (please print)

                                         Title:
                                               ---------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
    ----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------


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Infonautics, Inc.
August 20, 2001
Page 9


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         Eurocom Communications Ltd.
                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ Shaul Elovitch
                                             -----------------------------------

                                         Name: Shaul Elovitch
                                              ----------------------------------
                                                       (please print)

                                         Title: Chairman
                                                --------------------------------
                                                        (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
    ----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 10


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         FIBI Investment House Ltd.
                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ Zeev Feldman
                                            ------------------------------------

                                         Name: Zeev Feldman
                                               ---------------------------------
                                                        (please print)

                                         Title: C.E.O.
                                                --------------------------------
                                                        (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
   -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 11


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ M. Cooperman
                                             -----------------------------------

                                         Name: M. Cooperman
                                               ---------------------------------
                                                          (please print)

                                         Title:
                                               ---------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.



By: /s/ Van Morris
   -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 12


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ Graham Morris
                                             -----------------------------------

                                         Name: Graham Morris
                                              ----------------------------------
                                                          (please print)

                                         Title:
                                               ---------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.


By: /s/ Van Morris
   -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 13

          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ Elliot Noss
                                             -----------------------------------

                                         Name: Elliot Noss
                                               ---------------------------------
                                                          (please print)

                                         Title:
                                               ---------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.


By: /s/ Van Morris
   -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------

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Infonautics, Inc.
August 20, 2001
Page 14


          (L) This letter shall not be modified or amended, or any right
     hereunder waived or any obligation excused, except by a written agreement
     signed by both parties hereto.

                                         Very truly yours,


                                         ---------------------------------------
                                         [Please print name of entity, if any]


                                         By: /s/ Colin Campbell
                                             -----------------------------------

                                         Name: Colin Campbell
                                               ---------------------------------
                                                          (please print)

                                         Title:
                                               ---------------------------------
                                                          (please print)

Acknowledged this 20th day of August, 2001.

Infonautics, Inc.


By: /s/ Van Morris
   -----------------------------------------

Name: Van Morris
      --------------------------------------

Title: Chief Executive Officer and President
       -------------------------------------