I am testing on a remote server, not my local computer. I made the changes you suggested but still get the Transaction Not Completed message from the checkout_failure.php page along with the dump of the session variable. One question: what do you mean by requesting API Credentials? I'm assuming you mean the login info for the PayPal Paymens Advanced account. I'm using my clients live account, but the Transaction Processing Mode is set to "Test". Under PayPal Express Checkout->PayPal Sandbox email address: I have a valid sandbox email address and I'm using the fake VISA number associated with that buyer. I can't figure out what other settings may be wrong. Any clues here?