If you are getting a failed shipping quote request, or the quote is not what you think it should be the first thing you should do is put the full request and full response bindings for USPS onto the page you are seeing the bad results on. Here is the code that you will need to add to a page:
//WA eCart UPS Shipping Include
<?php echo "the full request is " . $_SESSION['eCart1_USPS_FullRequest']; ?><?php echo "<br/>The full response is " . $_SESSION["eCart1_USPS_FullResponse"]; ?>
If your cart is named differently then you will need to update the part of the code that says eCart1 to the name of your cart.
Put this code on the page where you would like to see this information, then run a test. The information in the full request and response should let you know exactly why you are getting the failed shipping or the amount of the shipping.