If you're just going to use the Amex gift cards at Costco, you could just get a Costco cash card in Canada

When using the Cash Card in the US, Costco shouldn't charge the 2.5% exchange rate either (but hard to verify as the receipt converts the remaining balance to USD too). Only 1 point per $ spent at Costco Canada I suppose, so depends how others value 1 Avios/Aeroplan