We've done this several times. I have DOUA and spouse purchases a single Cheers package once on board. You can purchase at any bar and bartender will call bar manager for approval. We have also purchased it at guest services, but they did not contact bar manager immediately, and told us to wait 30-60 minutes to order a drink, and we are not patient, so now, we purchase it at a bar and let the bartender get the approval right there so we can start ordering drinks immediately.