use this code for writing the full request and full response:
<?php
if(!session_id()) session_start();
echo "<p>USPS: <br />";
echo "Full Request: ".(isset($_SESSION["<cartName>_USPS_FullRequest"])?htmlentities($_SESSION["<cartName>_USPS_FullRequest"]):"n/a")."<br />";
echo "Full Response: ".(isset($_SESSION["<cartName>_USPS_FullResponse"])?htmlentities($_SESSION["<cartName>_USPS_FullResponse"]):"n/a")."<br /></p>";
?>
make sure to replace "<cartName>" with the name of your cart object. this will show the xml to help decipher the issue.