Possibly either. So they sell both? Any reason to think that it won't count towards $100 to get $20 Chase credit?
The costco cash cards are like buying anything from costco and wouldn't post as a cash advance or anything out of the ordinary.
Or walk into costco and buy one.
People are always looking for costco cash cards to buy as you need to be a member to shop in costco, hence with a costco cash card, that's a free entrance into the store and being able to shop. Many people will be willing to pay the full $100 as your doing them a favor they otherwise wouldn't of been able to buy themselves.