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.