UPDATE 2020: THIS PLUGIN HAS BEEN DISCONTINUED.
WordPress USPS Tracking Plugin allows you and your clients to track a shipment via USPS. It will work nicely with any WordPress configuration and does NOT depend on an e-commerce plugins.
Use WordPress eShop USPS Shipping Rates Plugin to replace eShop’s default shipping methods with live calls to United States Shipping Service.
eShop Shipping Extension framework overrides eShop’s default shipping methods. Add USPS support with WordPress eShop USPS Shipping Rates Plugin for real time shipping rates.
Please note that if you’re using WooCommerce, this is NOT FOR YOU! This works with eShop for WordPress only.