Since the Express Checkout error was saying the Return URL and Cancel URL were the problems I decided to try putting the full URL of those pages into the Checkout page.
That part worked. The Express checkout took me to PayPal, back to the confirm page and finally the Receipt page.
However... the Credit Card transaction still took me from the Confirm page back to the Cart page with the cart showing my items even though the transaction had been completed.
IS THERE A FIX FOR THIS PROBLEM?