@mia There is a small niche of people living on the Canadian border who this *might* impact. As far as I know, Canadians are the only ones who benefit from lower grocery prices by shopping across the border. I know the Bellingham, WA location is an example of a Costco that is bombarded by Canadians in search of cheap milk, though this might have died down a bit with the strengthening USD.
My guess is that the majority of these Canadians pay cash anyway due to forex fees on Canadian-issues credit cards, so the Visa/MC thing isn't an issue anyway.