Costco in the US used to only take American Express, and then only Visa (when they partnered with Visa) and now you can just use anything. In Iceland they take everything because the idea that they wouldn't is just a cultural headscratcher to a citizenship that has all of 3 banking options. I suspect they have a special business or "pro" membership that includes a mastercard option there? That was the old motivation before they gave up on it here.