The USPS changed their APIs on May 31, 2015, impacting shipping rate requests for international shipments to and from Canada. Some shipping rates will not be properly returned, and customers will not see all shipping options available to them. You can read more about the API change in a blog post written by WebShopApps, a Magento Partner.
To address this change, merchants who ship to and from Canada using USPS live rates will need to update their Magento site with the USPS API patch. Here are additional details:
Please upload the patch into your Magento root directory and run the appropriate SSH command:
sh patch_file_name.sh
Example: sh PATCH_SUPEE-1868_CE_1.7.0.2_v1.sh
patch –p0 < patch_file_name.patch
Once that is done, refresh the cache in the Admin under "System > Cache Management" so that the changes will be reflected. We highly recommend you test all patches in a test environment before taking them live.
For further instructions, see Installing a Patch for Community Edition.