You mentioned that when you test the site out you get the error. This error should be coming from your failure page. On this page you need to add in the code I provided on the page where you would like to see the information, it must be added into the code though, not design view.
You should then upload this file and any other files that you need to test the process out again. The USPS file that is included in the code should be in your WA_eCart directory already if you have the USPS shipping server behavior applied to your site. This same include should be present on your confirm page.
When you get to the failure page it should contain the full request and full response from USPS. If you can't see the message on this page you should view the source code to see if there is some xml there that you can post back. We will need to know what this information is to determine what the problem might be.