The USPS Shipping Method plugin for WooCommerce will get real-time rates from the UPS API. The plugin work with domestic and International package. Your USPS key must be for Production, contact USPS if you want them to enable it. Development key will not work.
*** USPS Shipping Method Changelog ***
2015.05.08 – version 2.1.8
* Fix minors bugs
Demo USPS Shipping method for WooCommerce
Download USPS Shipping method for WooCommerce v2.1.8