Since my client is a print shop and users would be submitting their photos to their site willingly (to print them), I think that there are no copyright issues here to be tackled.
I've played with a PP version that had a very rudimentary shopping cart, couldn't handle no more than one photo in the cart and the remove function didn't work. I don't know if my that version is outdated or not but I can't show it to my clients.
Is there any place where they can see the working part .. working? =) A demo of some sort?