As a PP said, the cash card generally amounts to 8-9% of the cruise fare. It costs $55 to join Costco, so factor that in to your decision. If you have a Costco near you, it's definitely something to consider. FWIW, some people here on the DIS have reported that they were able to redeem their cash card for cash at Costco customer service.
When you book onboard, you should designate Costco Travel at that time. That's simpler than booking directly with
DCL and then transferring. You can actually transfer to Costco Travel later, though, and still get the same cash card amount you would have received had you designated them when you made the OBB. Just be aware that you must transfer within 30 days of making the original reservation. If you do choose to name Costco Travel as your TA when you make the OBB, you'll need the following info:
Costco Travel
425-657-1900
Issaquah, WA
The downside to using Costco Travel is that the phone reps aren't always super knowledgeable about DCL. I've had to hold
their hands on occasion, but I know enough at this point that it's not an issue for me!