Your USPS key must be for Production, contact USPS if you want them to enable it. Development key will not work. The USPS Shipping method for WooCommerce will get real-time rates from the UPS API. The plugin work with domestic and International package.
Rename default shipping name to suit your needs. With the included auto update, future updates will be available directly from Wordpress interface.
Domestic
Priority Mail Express Mail Standard Post First-Class
International
Express Mail International Priority Mail International Global Express Guaranteed First Class Package Service
Changelog: v1.0.3 – April 25 2013
Added a way to change shipping methods name Added update from Wordpress Fixed weight Fixed First Class Parcel
v1.0.2 – April 24 2013
Fixed post to USPS API