I buy those too as just something I can pre-pay. Billy is right that they will deliver them the first night at dinner in the MDR. If you don't go to the Main Dining Room the first night, you can just go by there when they are open and the host/hostess will get them for you and you sign for them. If you get them as a gift from Carnival/Ocean Players Club, or someone sends them to you it works the same.
I think I know what JENSBEENCRUISIN's husband was talking about. It was a card that could be purchased on-board for 5 DOD's (Drink of the Day). It equaled to about a 20% savings if I recall correctly. They would punch the card after each use.