add the following to the checkout_failure page:
<?php
if(!session_id()) session_start();
echo("PayPal Full Request :<br />");
echo("<code>".isset($_SESSION["PP_DirectPayment_postedValues"])?$_SESSION["PP_DirectPayment_postedValues"]:""."</code><br /><br />");
echo("Paypal Full Response :<br />");
echo("<code>".isset($_SESSION["PP_DirectPayment"])?$_SESSION["PP_DirectPayment"]:""."</code><br /><br />");
echo("USPS Full Request :<br />");
echo("<code>".isset($_SESSION["WA_Store_Cart_USPS_FullRequest"])?$_SESSION["WA_Store_Cart_USPS_FullRequest"]:""."</code><br /><br />");
echo("USPS Full Response :<br />");
echo("<code>".isset($_SESSION["WA_Store_Cart_USPS_FullResponse"])?$_SESSION["WA_Store_Cart_USPS_FullResponse"]:""."</code><br /><br />");
?>
then test a credit card transaction. this will write the full request and ful response to the page for trouble shooting,
NOTE: The full request will contain sensitive info such as API credentials and CC#, block those values out when posting.