>I am looking for Shopping Cart Software to add this feature to our website. We are currently selling just a few products (software that we develop in house, replacement manuals, cd's) so we want something that is easy to use and maintain. Any suggestions?

If you don't mind a solution that's hosted on another server and paying service charges based on transactions (no monthly service charge), you might consider:

http://www.ccnow.com/overview.html