I believe the way it now works is that you pay $5/day for the sticker, which gets you unlimited sodas for the cruise. If you also want the souvenier mug, that costs an additional $5 on top of the per-day soda charge. In the past, the per-day soda charge got you the mug, too.
The KTTW card is short for "Key to the World" card, which serves as your boarding pass, room key, and on-board charge card. You affix the sticker to it so that the bartenders know that you have paid the unlimited soda charge. The soda charge is per person, I believe, so it would cost $5/person, per day for unlimited soda.
Note that soda is free in the dining rooms with meals, at least it used to be.