You will need to put the bindings for the full request and full response on the checkout failure page to see what is causing the error. Here is the code you will need to put on this page, you just insert this code where you would like to see the information appear on that page:
full request
<?php echo $_SESSION["WA_Store_Cart_USPS_FullRequest"]; ?>
full response
<?php echo $_SESSION["WA_Store_Cart_USPS_FullResponse"]; ?>