Way behind and might be missing something here, but Costco Travel only accepts Visa and MasterCard (
https://www.costcotravel.com/Info/Terms-And-Conditions). I know they just pass the payment to Disney, but they will only take those as payment. If you book directly and then transfer it to Costco, I guess you could get around it that way, but then it wouldn't be 5% category I suppose. Just thought I would share since I booked my cruise through them today and they definitely only offered the two options.
One other unrelated Costco Travel tip I learned today is that they will either keep you on hold while they call Disney or will offer call you back once they get them on the line. I was so grateful to stop listening to hold music and just wait for the call. It went well, but took 1 hour to get a Costco agent on the line, and 2.5 for them to reach Disney. I feel for travel agents right now.