Apologies for dredging up this old thread but I am having what seems to be the same problem
I am working on a client's site and just updated Powerstore from 4.5 to 4.5.3. I am requesting rates from FedEx. My problem seems to be exactly the same as BWINJ above (http://www.webassist.com/forums/post/145480/).
I have included debug code for the request and response on confirm.php. Each time I test, the request shows up 3 times and the response shows up 3 times. Rates are included in the response. However, the request is "SUCCESS" but fails because I get "0" for both the FedEx quote and response code.
I am using the latest version of "WA_eCart_FedEx_v10_PHP.php" from March 2014. (http://www.webassist.com/forums/post/170528/)