The store also has an unwritten policy regarding ... With these cards, non-members can shop at Costco, and they can even exceed the total value as long as they pay the remaining balance in cash ...
you can use a Mastercard or Discover card at Costco.com and via the Costco app. If you prefer to shop in-store at Costco with a credit card, then make sure you have a Visa-backed credit card.