Are you all using the module that says 'International Shipping (USPS)' in the admin->modules section? Or is this different? I am getting the same error:
An error occured with the USPS shipping calculations.
If you prefer to use USPS as your shipping method, please contact the store owner.
I have it set in 'test' mode. I'm going to try production mode after I get my USPS web tools account updated to production.