Merchant giftcards are not a cash equivalent. The merchant who sells it (Costco, in this example) buys the card for a discount from face value. It's the same as any other retail purchase.

Prepaid American Express, Mastercard, or VISA cards are typically sold at a premium to face value. They will be treated as a purchase if bought at a retail store, but if purchased at an American Express Travel Service Office (or other financial institution) they may be coded as a cash equivalent or even as a cash advance .