<PAGE>

                                                                   EXHIBIT 10.51

                                           April 13, 2001


PSC Inc.
PSC Scanning, Inc.
675 Basket Road
Webster, New York 14580-9787
Attention: Edward Borey, President and
           Chief Executive Officer
           Elizabeth J. McDonald, Vice President and
           Corporate Counsel

       Re:   PSC Scanning, Inc.


Ladies and Gentlemen:

      Reference is hereby made to the Securities Purchase Agreements dated July
12, 1996 (as amended, modified and supplemented, the "Securities Purchase
Agreements") by and among PSC Inc. (the "Holding Company"), PSC Scanning, Inc.
(the "Operating Company") and the institutional investors named therein.
Capitalized terms used but not defined herein shall be given the meanings
accorded to such terms in the Securities Purchase Agreements.

      Attached hereto as Annex 1 is a term sheet (the "Term Sheet") setting
forth our agreement on the terms of an amendment to or amendment and restatement
of the Securities Purchase Agreements. The parties hereto agree to promptly
prepare all documentation required in connection with such amendment or
amendment and restatement. Such documentation will not contain any economic
terms with respect to principal, interest, fees or financial covenants more
restrictive terms than those set forth in the Term Sheet. The holders of the
Notes hereby waive the Events of Default waived by them pursuant to the Consent
and Waiver Under Securities Purchase Agreements dated as of March 31, 2001
pending completion of final documentation. This letter agreement shall be
superseded and replaced upon completion of documentation in connection with the
amendment or amendment and restatement of the Securities Purchase Agreements
referred to above.

      By each party's signature below, each such party acknowledges that it is
duly authorized to approve the Term Sheet and agrees that the terms of the
Securities Purchase Agreements shall be deemed to be modified effective as of
the date hereof as set forth in the Term Sheet.
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      Please return executed counterparts of this letter agreement to Charles L.
Glerum and W. Brewster Lee at Choate, Hall & Stewart (fax no. 617-248-4000).


                                           Very truly yours,

                                           JOHN HANCOCK LIFE INSURANCE COMPANY
                                           (formerly John Hancock Mutual Life
                                           Insurance Company)


                                           By: _________________________
                                                                 (Title)


                                           JOHN HANCOCK VARIABLE LIFE
                                           INSURANCE COMPANY


                                           By: _________________________
                                                                 (Title)


                                           THE LINCOLN NATIONAL LIFE
                                           INSURANCE COMPANY

                                           By:   Delaware Lincoln Investment
                                                 Advisers, a series of Delaware
                                                 Management Business Trust,
                                                 Attorney-in-Fact


                                                 By: ___________________
                                                                 (Title)


                                           LINCOLN NATIONAL INCOME FUND,
                                           INC.


                                           By: _________________________
                                                                 (Title)
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                                           SECURITY-CONNECTICUT LIFE
                                           INSURANCE COMPANY


                                           By: _________________________
                                                                 (Title)


                                           THE EQUITABLE LIFE ASSURANCE
                                           SOCIETY OF THE UNITED STATES


                                           By: _________________________
                                                                 (Title)
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ACKNOWLEDGED AND AGREED


PSC INC.


By: __________________________


PSC SCANNING, INC.


By: __________________________


PSC AUTOMATION, INC.


By: __________________________


INSTAREAD CORPORATION


By: __________________________


PERCON INCORPORATED


By: __________________________