OK, I found out why the PayPal payment is failing. I kept your code fix to prevent orders from processing on an expired session. The reason why the payments are failing has to so with multiple items in the shopping cart. In 5 recent payment tests, all the payments worked for single item purchases and all the payments failed for multiple items purchases. In other words, a successful PayPal payment is only possible if one item is purchased. Put 2, 3, or more items in the shopping cart and the checkout fails. This is the issue. So, is this a fixable problem? I certainly hope so.