USPS API Retirement
In May 2011, USPS announced that Web Tools Domestic Rate Calculator API using Rate and RateV2 will be retired on March 31, 2012. If you are still using Rate or RateV2 APIs, they will will result in error message responses.
If your USPS shipping is not working then that is probably the cause.
If you are trying to upgrade yourself using the latest published Add Ons and it is not working for you then we recommend using version 5.2.2 with some patches. Contact us if you want a copy of our version and some how to upgrade notes.
And remember, if you are using phpV5.3 you’ll probably need to make some changes to handle it.
Please contact us for an upgrade.