Just in case you got an order in your Magento store recently that has an incorrect charge for USPS shipping, you may be experiencing a bug related to an update in USPS on Sunday, May 31, 2015. The US Postal Service (USPS) released an update to their rates and services making origin zipCode required for Priority Mail International to Canada.
The result is that orders to Canada from the US are only charged the handling fee handled in Magento backend. The fix is listed in Magento Stack Exchange: http://magento.stackexchange.com/questions/69606/usps-1-9-1-1-priority-mail-international-broken
Best of luck!