I have successfully installed oscommerce for couple of clients before and totally customized it, but I have a client now that already has a website built with product listings.. all in html. All what he needs is the actual shopping cart, and payment system (Credit cards). So I would like to provide that for him but... He has a budget of $200-$300 for this and I don't want to use oscommerce or similar ecommerce cms because as I said, he already has a nice static website but just needs the backend of it to work.

So I'm looking for a shopping cart script out there that is neat and can be somehow not too complicated to incoperate into the current website in any scriptting language but PHP is preffered. What do the experts here propose?