Ok, so on the failure page, edit the shipping error message to include the error code and description from USPS:
<?php @session_start(); ?>
<p>Your transaction could not be completed due to a problem occurring while calculating shipping.<br>
Error Code:<?php echo $_SESSION["eCart1_USPS_ErrorCode"]; ?><br>
Error Description: <?php echo $_SESSION["eCart1_USPS_ErrorDescription"]; ?><br>
that will give some additional info on why the shipping rate is failing.