Possibly interesting to someone: Costco in Canada no longer has that Amex relationship. Instead, they are associated with Capital One and their Mastercard. There's a page on the costco.ca site with a heading of "Credit Cards Accepted at the Warehouse" that says:

The following cards are accepted in our Canadian warehouse locations and at our gas stations:

MasterCard Debit cards Costco Cash Cards

I didn't even think to try last time I was at Costco, but this does seem to imply we can use our normal Mastercard credit card there now?