Flexible Shopping Cart (was: Shopping Carts)

I've sent a message some hours ago and committed the mistake of including it inside of an already started thread, don't know if someone read it. The details are in that thread but basically what I am looking for is a shopping cart that has flexibility with respect to the graphical layout of elements and their appearance as well as the ability to extend the framework with custom functionalities. Don't have good experience with oscommerce because of the way the layout is set with tables, it doesn't give much freedom and couln't find good docs about it. Saw Zen Cart and it looks promising but have no experience on it. Listen to your suggestions. Thanks again.