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.
Here is when you should avoid shopping at Costco at all costs unless you truly like crowded aisles, long checkout lines and fewer products on the shelves. Sure, you don't want to get all wet, but ...