If you are working on this in a ticket you will get a response back to the issue there. If you have not done so already you should put the USPS responses on the checkout failure page so you can see what the error is. Here are the various responses that you will need to put into the code somewhere on the page to see what the error is:
php:
<?php echo $_SESSION["WA_Store_Cart_USPS_Quote"]; ?>
<?php echo $_SESSION["WA_Store_Cart_USPS_ResponseCode"]; ?>
<?php echo $_SESSION["WA_Store_Cart_USPS_ErrorCode"]; ?>
<?php echo $_SESSION["WA_Store_Cart_USPS_ErrorDescription"]; ?>
<?php echo $_SESSION["WA_Store_Cart_USPS_FullRequest"]; ?>
<?php echo $_SESSION["WA_Store_Cart_USPS_FullResponse"]; ?>
