I book onboard for the $200 onboard credit, and name Costco as the TA. With the price of the Concierge rooms, it really pays off to get the 8.5% of your cruise cost as a Costco cash card. I also upgraded to the executive membership and the 2% additional return at the end of the year. The Cash card alone pays for more than the upgraded membership.
Our next 2 cruises with only 2 people in the cabin, we get a Costco Cash card for $1045 and an additional $220 at the end of the year.
If not booking onboard I book directly with
DCL on opening day to get the specific cabin I want, then transfer it to Costco.