There was a USPS update released for Magento (SUPEE-6237). It pertains to shipments to Canada. Our team is working on making sure all websites using USPS for shipping are being updated.
USPS API Update May 31st 2015 – What you need to know (webshopapps.com)
USPS has updated their shipping rate API on May 31st 2015. As we always do, we’ve reviewed these updates and analyzed their potential impact on merchants running various versions of Magento. There are some impacting changes for merchants shipping to or from Canada, or those using Dimensional shipping.
Within the API update, the only change with impact to Magento USPS users is:
Changes to the rate request for international shipments to/from Canada, requiring an Origin post code
Some rates will no longer be returned for shipments between USA and Canada, this will impact shipping options displayed to your customers.
WHAT SHOULD YOU DO?
If you ship to or from Canada using USPS live rates, you’ll need to update your Magento site as soon as possible.