I need to add shopping cart functionality to my image browsing site, so users can order a high res version of an image they like. I've looked at some cart software on sf.net but they all seem to have way too many features than what I need. My web site already has a catalog listing and searching of images.
I need a shopping cart software that would let me enter an 'Add to cart' link next to each image, and then handle the cart management, checkout, and user authentication. It should be open source since I'd probably need to customize it, and run on Linux with php and postgresql. Is there any thing like this out there? Or can one of the big ones like zen cart be easily customized and used in this fashion?

Thanks for any suggestions.