Well... everything seemed to go pretty well, until I tried purchasing a subscription. When (as a customer) I click the link to purchase, I go to:
https://www.paypal.com/cgi-bin/webscr
and that page displays:
Error Detected
The link you have used to enter the PayPal system is invalid. Please review the link and try again.
Any clues as to how to fix this?
I checked the code over as best as I can, and I can't see where the problem might be.