In order to activate the new USPS shipping feature for Marketpress you need to do one of 2 things. The email you received about testing is pretty much not needed except your username, password and the contact information to the USPS tech support. After 40 minutes on hold I discovered that if using Marketpress you don’t have to run any tests (no testing on your part needed) as they have apparently already taken care of this process when they wrote the code.
So all you need to do is either call 1-800-344-7779 and let them know you are using Marketpress Plugin and they will transfer you to their production server. (only took 3 minutes after I talked to someone)
Or you can email them at. email@example.com
In either case have the email you registered with the USPS API request and your Username that they emailed you.
Hope this helps everyone!