I bought such a package on the Dawn last month, at a considerable savings over the wine list price. They offered three levels, from their least expensive wines to moderately priced bottles (around $40 on the wine list) and you could select 2, 4 or 6 bottles. The more expensive bottles on the wine list were not available in a package.
Top 10 Contributor
Yes, you can buy in onboard the ship. Usually when you board the ship, they have a desk setup (usually near the guest services desk) where they will sell you the package. The last time I sailed, they even had a desk at the port just after you check-in and get your room key cards.
Number One Contributor