Seems you're missing out on part of the basic paypal function. Somehow you've made it open in the same window. That's ok, but when I go back to the main window to buy another product, it opens the new product, then paypal in the same window, leaving 2 paypal checkout windows open. One with 1 product, 1 with 2. If I checkout on either, it has 2 products.
Why not keep everything in the same window except for paypal shopping cart in it's own window. When you add more product it updates the shopping cart in the other window.
I've been dealing with paypal too and so far the client has been satisfied at
http://www.hownet.com