DrByte,
After further testing I found that I can consistently make a purchase, once at the Paypal site, using a credit card and the "click here to return to the jwbooks site" button takes me back to the store every time. The page not found error only occurs when I use my Paypal account to pay and Pypal initiates the redirect back to the store.

What is different between the two forms of payment or even between the two return methods???