Again, unless it's changed, when you make an in-store return at Costco they make you put the credit card into the card reader in order to process the refund. Since the in store card readers won't process a Mastercard credit card or Discover credit card you'll be told to insert a Visa card. If you insert a Visa *debit* card you'll get cash into your account.

I have only done this 2 or 3 times and always for a legitimate reason as I don't want to get banned from Costco.