The testing url only lets specific test xml get a valid reponse. Here's what I did, put a break in HttpRequestHandler.vb Line 57. It should be the post function, when the breakpoint is made, change the package portion of the xmlpostdata string to match the 2 test requests on this page http://www.usps.com/webtools/htm/Rates-Calculatorsv1-0.htm#_Toc87669069 section 3.3 step 1
You can put a break in USPostalServiceShippingProvider.vb line 92. and view the contents of rateXML, to see if they match the reponse provideded on the usps page section 3.3 step 3
Of course, that is just for testing, you can also just send an email to usps and tell them that you use the dashcommerce package and they will switch you to the live server url.