<PAGE>

                                                                  EXHIBIT 10.7.1
                         [LETTERHEAD OF CLEARCOMMERCE]


December 31, 1998

Mr. Bruce Shirey
Vice President- InternetSolutions
Card Services International
4227 Sunrise Blvd, Suite 200
Fair Oaks, CA 95628

Subject: License Agreement Addendum A- Extension of exclusivity and additional
product purchases

Dear Bruce:

During our meeting of December 15-16/th/ and conference call today, we discussed
CSI's desire for an extension of the CSI exclusivity on CyberPoint (Virtual
LinkPoint). In exchange for ClearCommerce extending the exclusivity period, CSI
agrees to purchase an additional number of software licenses. The following are
the terms and scope of what was agreed upon, with this document becoming an
addendum to the above agreements between CSI and ClearCommerce upon your
signature below.

In addition, for an order placed by Dec. 31, 1998 for [*], CCC has agreed to the
following:

  .  CSI has the right to order additional SSL API or CyberPoint (Virtual
     LinkPoint) (licenses by February 28, 1999 at an agreed to price of [*]
     each). Based on [*] licenses this could result in savings to CSI of [*].

  .  CCC agrees that the LinkPoint HTML and Virtual LinkPoint product will be
     combined to represent a single product for which one (1) license fee will
     be paid vs the 2 licenses currently charged. Based on 16,500 merchants this
     could represent savings to CSI of over [*].

  .  [*]

Exclusivity
-----------

The following supercedes Section 17 of the License and Service Agreement:

CCC will grant CSI a period of exclusivity for CyberPoint Client Side product as
currently delivered as of (12/22/98) for one (1) year in the United States
commencing December 31, 1998. In addition, CCC will grant one-year exclusivity
from the delivery of each new feature(s) or products that CSI pays CCC to
         ----------------------------------
develop under a Professional Services contract.  Additionally, a 90-day
exclusivity for feature(s) that CCC planned to deliver in the future, but CSI
funded to accelerate the delivery date.

     A.  CSI and CCC agree that exclusivity does Include:

         1.   The CSI proprietary packaging of the functions and features into
              products known as Cyberpoint and Virtual LinkPoint.

         2.   Any future CSI enhancement or individual components that comprise
              CyberPoint and Virtual LinkPoint.

         3.   Any integration of features discussed in (B.3) into the CyberPoint
              product would be CSI specific and thus a CyberPoint and Virtual
              LinkPoint like Product. These features would not be relicensed by
              CCC until the end of the mutually agreed upon exclusivity period.

         4.   The development of a specific CSI "Virtual/Physical" LinkPoint
              like terminal interface would be covered under the one year
              exclusivity agreement and specific to CSI's "Virtual/Physical"
              LinkPoint implementation.


[*] Certain information on this page has been omitted and filed separately with
    the Commission. Confidential treatment has been requested with respect to
    the omitted portions.

<PAGE>

                            [LOGO OF CLEARCOMMERCE]


     B.  CSI and CCC agree that exclusivity does not Include:

         1.   CSI ideas, concepts, etc. that is considered to be in the public
              domain. (For example, the concept of a point of sale terminal is
              in the public domain.

         2.   The CCC developed Internet point of sale terminal (sale manager).

         3.   CCC planned enhancements. (For example: Recurring billing, Check,
              ACH, Auto merchant set-up, Debit card, Purchase Order, Smart Card,
              Magnetic card reader support and Purchasing Card Level II and
              Level III support, along with the appropriate fraud and reporting
              enhancement features to the Hosting and Merchant Engine and thus
              would be offered to the licensees of the Hosting and Merchant
              Engine)

         4.   Market or third party requirements to CCC for development of
              common interfaces or other software.

         5.   Any written requirement and specifications for CCC Professional
              Services by another company that may be the same or similar to CSI
              products providing that CCC does not use any CSI specification or
              CSI code developed on behalf of CSI by CCC while an exclusivity
              period is in effect.

         6.   Any acceleration of CCC planned enhancements by any CSI funded
              development of a product or feature(s), and if it causes CCC to
              delay any planned release date of same product or feature(s). (For
              example, if CSI accelerates the development of Purchasing Card
              level II and/or level III, CCC will provide CSI a three-month
              exclusivity period as long as CCC did not plan to release the CSI
              requirement within that same three-month exclusivity period.

CCC may include the CyberPoint product and new enhancements in bids to
ClearCommerce prospective customers during the exclusivity period with CSI's
prior written approval to each specific prospect. Any derivative thereof, and/or
any new functionality can only be interfaced to a CCC Hosting Engine, and must
include a per copy license fee paid to CCC as referenced in this CSI/CCC
agreement.

License Purchases
-----------------

On Page 5 of the License Agreement, the second page of the Product and Pricing
Schedule, add the following paragraph to the Payment Terms section:

"CyberPoint, HTML Wrapper, and Storefront SSL API licenses: CSI will purchase
additional [*] licenses for [*] each by Dec 31, 1998 totaling [*]. Payments will
be scheduled as follows: [*] on January 1, 1999 [*]. On April 1, 1999, [*] on
July 1, 1999, and [*] on September 30, 1999.

Contract Terms
--------------

The terms and conditions of this letter are hereby incorporated into the License
and Service Agreement and Its attachments ("License Agreement"), executed June
30, 1998, and become an addendum to the License Agreement ("Addendum A"). The
terms of the License Agreement hereby apply to this Addendum A upon execution by
CSI. Any terms specified herein that Conflict with the terms of the License
Agreement shall supersede the terms in the License Agreement, including the
attachments. This Addendum A only applies to those products specified herein and
to the extent noted.

This proposal for Addendum A by ClearCommerce is valid until December 31, 1998
and must be executed by CSI by that date. CSI's signature below will represent
CSI acceptance of these terms and order for the [*] SSL API, CyberPoint
(Virtual LinkPoint) licenses.

[*] Certain information on this page has been omitted and filed separately with
    the Commission. Confidential treatment has been requested with respect to
    the omitted portions.

<PAGE>

Execution of this contract addendum by the parties signifies the agreement of
the parties to the terms set forth herein with an execution of the last
signature below.

ClearCommerce Corporation               Cardservice International
-------------------------               -------------------------

BY: /s/ Michael S. Gajeda               BY: /s/ Caesar Berger
    ----------------------------            ----------------------------

PRINTED NAME: Michael S. Gajeda         PRINTED NAME: Caesar Berger
              ------------------                      ------------------

TITLE: Vice President, CFO              TITLE: Senior Vice President
       -------------------------               -------------------------

DATE: 12/31/98                          DATE: 12.31.98
      --------------------------              --------------------------