ok, thanks. Add this code to the checkout failure page after the body tag:
<?php
if(!session_id()) session_start();
echo "<p>USPS: <br />";
echo "Full Request: ".(isset($_SESSION["WA_Store_Cart_USPS_FullRequest"])?htmlentities($_SESSION["WA_Store_Cart_USPS_FullRequest"]):"n/a")."<br />";
echo "Full Response: ".(isset($_SESSION["WA_Store_Cart_USPS_FullResponse"])?htmlentities($_SESSION["WA_Store_Cart_USPS_FullResponse"]):"n/a")."<br /></p>";
?>
this will write the full request and full response form usps to the page to help troubleshoot the issue.