the failure page is the checkout_failure.php page. it is at the root of the power store files.
add that code to the checkout_failure.php page and make sure it is uploading to the server.
you should at least see:
FedEX:
Full Request:
Full Response:
on the page if the change is being uploaded to the server, if you do not see any change, then the change is not making it to the server.