Come Sept. 8, some $20 billion’s worth of U.S. goods will be subject to Canadian counter-tariffs of up to 50%. They follow the 50% duties imposed by Washington on Canadian goods on Aug. 22. Yet U.S. President Donald Trump’s renewed focus on Canada should be no surprise, after Washington declined to extend the U.S.-Mexico-Canada trade agreement in July.  For four decades, Canadian commercial life has been organized around the belief that access to the U.S. market was a constant, rather than a variable. That assumption no longer holds, whether or not tariffs are here to stay.  So where should Canadians…