Apparently USPS will start accepting mail for Canada again tomorrow. This taken from www.usps.com:
Canadian Postal Employees Return-To- Work Tonight - USPS To Begin Accepting Mail Destined For Canada Tomorrow
The Canadian government passed back-to-work legislation Sunday night. Canadian Postal employees will return to work beginning June 27 at 8:30 p.m. USPS will begin accepting mail for Canada starting June 28. Mail held in the USPS network since the work stoppage began is being released and transported into Canada.
Due to the large volumes of mail held by countries, customers may experience slightly longer transit times. USPS is working closely with the Canadian Postal Service to hold delays to a minimum.
Meanwhile, Global Express Guaranteed (GXG) still is available for delivery into Canada and remains the best option to avoid delays in service. GXG is available at thousands of retail locations across the country or through Click-N-Ship at usps.com.