There are many PayPal Shopping Cart Tutorials on the internet, but why are you building your own from scratch?  

There are many things to consider when building a PayPal Cart using a tutorial. First, the most important thing to consider is that the tutorials usually cover some very important topics briefly.  For security reasons alone, why try to build one yourself?  This is just one example, but is a huge issue for many is overlooking things like SQL injection or Cross-Site scripting vulnerabilities.

Do you really want to take chances on building this yourself.  What if you misunderstood something or completely overlooked something?  Try PDshop, which is already integrated with PayPal Payments Standard, PayPal Payments Pro, and PayPal's IPN (also known as Instant Payment Notification)