USPS made a change : "Origin ZIP Code required for Priority Mail International to Canada"

It was reported in the nopCommerce forums that USPS International Shipping stopped working

In fact, only international shipping to Canada broke, because...

May 31, 2015 USPS Web Tools will be implementing modifications and additional features to the U.S. Postal Service APIs. The following changes may especially impact shipping systems:

- Origin ZIP Code required for Priority Mail International to Canada
- Modified special services
- Modified service IDs
- Modified available mail classes for Merchandise Return Services

This link is a .zip file containing the fixed USPS plugins for nopCommerce versions 3.30, 3.40, and 3.50

You really only need to replace the file Nop.Plugin.Shipping.USPS.dll but I've included everything - source code, and no source versions.   Copy .dll from the zip file's \USPS Shipping\nopCommerce 3.xx\Shipping.USPS folder to your \Plugins\Shipping.USPS folder.  After you copy it to your plugin folder, be sure to "Reload list of plugins", or restart your web app.