If you have enabled shipping it should be based on the weight in the cart, if not all of your items will have shipping then the server behavior for this might need to have the trigger modified to check for the weight of the cart.
On the failure page that you are getting to you should add in the USPS full request and full response bindings to the page. Here is the code that you can add to the page:
//WA eCart Include
The full USPS request is: <br/><?php echo $_SESSION["WA_Cart_Coupons_USPS_FullRequest"]; ?><br/><br/>
The full USPS response is:<br/>
<?php echo $_SESSION["WA_Cart_Coupons_USPS_FullResponse"]; ?>
Run a test after adding this code to see what the error is, if you have any trouble with it post back with what you get minus any of the sensitive details.