I have encountered the same problem.
When I looked in the code,
in shipping/shipping.php on line 309 it tries to call a function
wpscShippingAPICheckIfServiceEnabled_USPS
but that function is not in any files.
May be the some file for USPS is missing from the build?
Or may be there needs to be some else clause?
jquindlen I request you to take a look there.
