Yes, I have booked 2 cruises with Costco, one Carnival and one Royal. Costco sent my confirming email with the cruise line number. I logged into both Carnival and Royal websites with my given cruise number, and with both I can order stuff. Carnival’s cruise is closer, so I get reminded from Carnival I can book all this stuff. Royal’s cruise is over a year away, so only thing I can book ahead is Perfect Day stuff, and of course merchandise to buy, but cannot book anything else until closer.
And cute, I have both as an app on my phone, and they count down to cruise day just like Disney’s one does.
Rest easy, you can book stuff to do on the cruise. Only thing is, if you want changes, or want to pay, put requests in, have to go through Costco.
And I am very much looking forward to the gift card I will get after each cruise.
Hope this helps.