USPS - API Authorization failure. RateV4 is not a valid API name for this protocol

Getting error: "USPS API Authorization failure. RateV4 is not a valid API name for this protocol" when trying to get real time rates.

I came to know that USPS's TEST server is using the older API method (RateV2), so the testing of RateV4 is not possible with TEST server.

Solution to USPS error:

The solution is very simple.
Just move your USPS account to the production server.