I decided to do this as a page, rather than a post, so here’s the link to the code that I use for my homemade Paypal Shopping Cart for photos.
If you find your eyes glazing over while you read it – then hit the back button quickly.
If you are a web designer, then I don’t think it’s all that complex. If you’re not – then it will look like a lot of work or worse, a reinvention of the wheel. Though when I first wrote this I don’t think this particular wheel existed yet.
There are several paypal cart plugins for WordPress at this point so you might start with one of them instead. On the other hand, this is actually pretty simple, and gives you flexibility that you might find hard to achieve with someone else’s plugin. Shipping calculations and if I wanted to charge taxes – that sort of stuff is handled by how Paypal is set up, not with the cart.