FEATURE 2014.6 : Support for USPS Retail/Online/Plus rate types

We've added support for USPS Retail/Online/Plus rate types.

This feature allows you to set up USPS rate type option when get shipping rate from USPS. Retail: Retail rates given at the Post Office. Online: Online pricing is a discounted rate offered to USPS customers who buy and print shipping labels online. Plus: Commercial Plus pricing is a program offered by the USPS that provides highly discounted postage rates for high volume shippers.