We always book through Costco. We usually end up calling because nowadays we are always booking with a placeholder, and you can't do that online. I call them, tell them exactly what I want, and then they put me on hold and call Disney to book it. As someone else said, since you aren't booking with a placeholder, you should be able to do it online. That said, not all of the available rooms show up on the Costco website. If you see a specific room that you want on the Disney site that is not available on the Costco site, you will need to call Costco and then they will call Disney. If you see a room on the Costco site that you are happy with, you can book it online.
FOR YOUR NEXT CRUISE AFTER THE ONE YOU ARE TALKING ABOUT BOOKING NOW: If you are planning to buy a placeholder during your cruise, just make sure you keep Costco as the travel agent (I can't remember how to do this in the app - it may be automatic if your current cruise was booked through Costco). That way when you call Costco to book the next cruise, it should be in their system already. If Costco isn't assigned as the travel agent, they won't be able to book it with the placeholder. I think someone said something about transferring it to Costco - that's a hassle and has to be done within 30 days of booking the placeholder. Much easier to just make sure Costco is assigned when you buy the placeholder.