There is no advantage to buying the soda package before the cruise. If you buy it before the cruise, you will have to pre-pay for it. If you purchase it onboard, the cost will be added to your onboard expenses.
Onboard the ship, they usually have a table setup (either on the deck where you board or next to the pool) where they will sell you the package. If you don't find it, just ask at the guest services desk or at any of the bars around the ship.
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