<PAGE>

                                                                   EXHIBIT 10.13



                              September 21, 1999


Mr. Brian Powley
25 W. 85/th/, Apt. #2B
New York, NY 10024


              Re: Terms of Employment Offer with MediaPlex, Inc.
                  ---------------------------------------------


Dear Brian:

     MediaPlex, Inc, (the "Company"), a wholly owned subsidiary of Internet
Extra Corporation ("IEC"), is pleased to offer you the position described below.
This letter sets forth the terms and conditions of your employment with the
Company.


I.   Description of Employment Position and Responsibilities. You will serve in
     -------------------------------------------------------
     the position of Vice President of Sales. By executing this agreement, you
     agree to assume and discharge such duties and responsibilities as are
     commensurate with this position and such other duties and responsibilities
     that are assigned to you from time to time by the Company's Board of
     Directors, Chief Executive Officer, or your then supervisor. During the
     term of your employment, you shall devote your full time, skill and
     attention to your duties and responsibilities and shall perform them
     faithfully, diligently and competently. In addition, you shall comply with
     and be bound by the operating policies, procedures and practices of the
     Company in effect from time to time during your employment.

II.  Employment Considerations.
     -------------------------

     2.1  At-Will Employment. You acknowledge that your employment with the
          ------------------
     Company is for an unspecified duration that constitutes at-will employment,
     and that either you or the Company can terminate this relationship at any
     time, with or without cause and without notice.

     2.2  Employment Eligibility. Your employment is contingent on your
          ----------------------
     submission to the Company of satisfactory original documentation to verify
     your identity and eligibility for employment in the United States.
<PAGE>

Mr. Brian Powley
September 21, 1999
Page 2 of 5


III.   Compensation.
       ------------

       3.1     Base Salary. In consideration of your services, effective October
               -----------
       11, 1999, you will be paid an annual base salary of $170,000 (One Hundred
       Seventy Thousand Dollars and no Cents), payable no less frequently than
       on a monthly basis in accordance with the Company's standard payroll
       practices. Your base salary, in conjunction with your performance
       evaluation, is normally reviewed annually by the appropriate management
       of the Company.

       3.2     Incentive Compensation. In addition to your base salary, you may
               ----------------------
       be entitled to participate in such incentive compensation plan that may
       be adopted by the Company in its sole discretion. You understand that the
       adoption of any such plan, the eligibility and measurement criteria and
       all other terms shall be at the sole discretion of the Company. You will
       also be eligible to receive a bonus of up to 20% (Twenty Percent) of your
       Base Salary upon the achievement of certain milestones to be established
       by your supervisor within thirty (30) days of your date of commencement
       with the Company.

       3.3     Termination. If your employment is terminated by the Company for
               -----------
       any reason, with or without cause, or if you resign your employment
       voluntarily, no compensation or other payments will be paid or provided
       to you for periods following the date when such a termination of
       employment is effective, provided that any rights you may have under the
       benefit plans of the Company shall be determined under the provisions of
       those plans. If your employment terminates as a result of your death or
       disability, no compensation or payments will be made to you other than
       those to which you may otherwise be entitled under the benefit plans of
       the Company.

IV.    Additional Benefits.
       -------------------

       4.1     Health Insurance/Vacation/Benefit Plans. You will be entitled to
               ---------------------------------------
       receive the standard employee benefits made available by the Company to
       its employees to the full extent of your eligibility therefor. You shall
       be entitled to two (2) weeks of paid vacation per year; the terms and
       conditions of your vacation benefits shall be in accordance with the
       Company's vacation policy in effect at that time. During your employment,
       you shall be permitted, to the extent eligible, to participate in any
       group medical, dental, life insurance and disability insurance plans, or
       similar benefit plan of the Company that is available to employees
       generally. Participation in any such plan shall be consistent with your
       rate of compensation to the extent that compensation is a determinative
       factor with respect to coverage under any such plan.
<PAGE>

Mr. Brian Powley
September 21, 1999
Page 3 of 5


     4.2   Reimbursed Expenses. The Company shall reimburse you for all
           -------------------
     reasonable expenses actually incurred or paid by you in the performance of
     your services on behalf of the Company, upon prior authorization and
     approval in accordance with the Company's expense reimbursement policy as
     from time to time in effect.

     4.3   Stock Options. Pursuant to Board approval, and under the terms and
           -------------
     conditions of the Company's Stock Option Plan and Stock Option Agreement,
     including the stock vesting provisions contained therein, you will be
     granted an option to purchase 40,000 (Forty Thousand) shares of common
     stock of the Company. The Company Stock Option Plan, including the Stock
     Option Agreement, will be sent to you separately.

V.   Intellectual Property Rights/Confidential Information.
     -----------------------------------------------------

     5.1   Valuable Trade Secrets. You agree that the Company is the owner of
           ----------------------
     valuable trade secrets, client, vendor, customer and contractor lists and
     other confidential and proprietary information. As such, you agree that
     your employment is contingent upon your execution of, and delivery to, the
     Company of a Confidential Information and Invention Assignment Agreement
     ("Intellectual Property Agreement") in the standard form utilized by the
     Company.

     5.2   Additional Confidential Information. You agree to maintain the
           -----------------------------------
     confidentiality of all elements of this Agreement, agreeing to an absolute
     prohibition on any disclosure or use of such information in any fashion,
     with the exception of discussions with your supervisor or myself, and
     Messrs. Jon Edwards and Tim Favia. As you are well aware the maintenance of
     confidentiality of this kind of information is critical to our
     organization.

     5.3   Equitable Remedies. I agree that it would be impossible or inadequate
           ------------------
     to measure and calculate the Company's damages from any breach of the
     covenants set forth in this Section as well as Sections 1, 2, 3, and 5 of
     the Intellectual Property Agreement. Accordingly, I agree that if I breach
     any of such Sections, the Company will have available, in addition to any
     other right or remedy available, the right to obtain an injunction from a
     court of competent jurisdiction restraining such breach or threatened
     breach and to specific performance of any such provision of this Agreement.
     I further agree that no bond or other security shall be required in
     obtaining, such equitable relief and I hereby consent to the issuance of
     such injunction and to the ordering of specific performance.

VI.  Non-Competition/Conflicting Employment. You agree that, during the term of
     --------------------------------------
     your employment with the Company, you will not engage in any other
     employment, occupation, consulting or other business activity directly
     related to the business in which the Company and/or its customers are now
     involved or become involved during the term of your
<PAGE>

Mr. Brian Powley
September 21, 1999
page 4 of 5


       employment, nor will you engage in any other activities that conflict
       with your obligations to the Company.

VII.   Arbitration. Except as otherwise provided in the Intellectual Property
       -----------
       Agreement described above, it is agreed that any dispute or controversy
       arising out of or relating to any interpretation, construction,
       performance or breach of this Agreement, shall be exclusively settled by
       arbitration to be held in San Francisco County, California, in accordance
       with the rules then in effect of the American Arbitration Association.
       The arbitrator may grant injunctions or other relief in such dispute or
       controversy. The decision of the arbitrator shall be final, conclusive
       and binding on the parties to the arbitration. Judgement may be entered
       on the arbitrator's decision in any court having jurisdiction. Each party
       shall pay one-half of the costs and expenses of such arbitration, and
       each of us shall separately pay our counsel fees and expenses.

VIII.  General Provisions.
       ------------------

       8.1  Governing Law. This offer letter will be governed by the laws of the
            -------------
       State of California, applicable to agreements made and to be performed
       entirely within such state.

       8.2  Entire Agreement. This offer letter sets forth the entire agreement
            ----------------
       and understanding between the Company and you relating to your employment
       and supersedes all prior verbal discussion and written agreements between
       us. Any subsequent change or changes in your duties, salary or other
       compensation will not affect the validity or scope of this agreement. Any
       change to the at-will term of this agreement must be executed in writing
       and signed by you and the President of the Company.

       8.3  Successors/Assigns. This agreement will be binding upon your heirs,
            ------------------
       executors, administrators and other legal representatives and will be for
       the benefit of the Company and its respective successors and assigns.
<PAGE>

Mr. Brian Powley
September 21, 1999
Page 5 of 5


     Please acknowledge and confirm your acceptance of this letter by signing
and returning the enclosed copy of this offer letter, and the Confidential
Information and Invention Assignment Agreement as soon as possible. If you have
any questions about this offer letter, please call me directly.

                                MEDIAPLEX, INC.


                                By: /s/ Gregory R. Raifman
                                    ___________________________
                                    Gregory R. Raifman
                                    Chief Executive Officer


ACCEPTANCE:

     I accept the terms of my employment with MediaPlex, Inc. as set forth
herein. I understand that this offer letter does not constitute a contract of
employment for any specified period of time, and that my employment relationship
may be terminated by either party, with or without cause and with or without
notice.

Mr. Brian Powley

/s/ Brian Powley
_______________________

9/24/99
_______________________
Date