One of the shopping cart systems we utilize often is OSCommerce, for many reasons. One, it's been around a long time and is very stable and there are a lot of plugin contributions readily available. Two, it's open source. And three, we work regularly with an amazing OSCommerce programmer - Monika, who recently wrote this book.It does a great job of providing easy to understand instructions on how to modify the OSCommerce shopping cart. I recommend it heartily and if you are interested in customizing OSCommerce - this is a must have book.
