Mark, did you use API=RateV4 in your test app?
I believe that the problem is that your account doesn't have permissions to use API=RateV4.
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RateV4 is not applicable in Testing server, therefore you won't be able to obtain any response. And, in regards to RateV4 in Production server, you will need to obtain permission from ICCC, USPS.
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There is the solution for the same problem with USPS in X-Cart 4
http://help.x-cart.com/index.php?title=X-Cart:USPS#USPS_error:_Authorization_failure._You_a re_not_author...to_this_server.
And some related issues from stackoverflow.com
http://stackoverflow.com/questions/15236053/usps-shipping-calculator-api-not-returning-valid-response
http://stackoverflow.com/questions/7885269/response-from-usps-rate-calculator
Please contac the Internet Customer Care Center (USPS) and ask them to switch your profile to allow you access to the production server.