When we were on our 7 day cruise, we purchased the mug. You automatically got a sticker with the mug. The mug was $5 extra, thought it would be a nice sovenier. If you wanted a soda, you showed your card with the sticker and they gave you a soda. With the mug, it was a bit odd. When you brought up your mug, they didn't ask to see your card with sticker, but they wouldn't fill the mug for you, they gave you your soda in a cup, then you dumped it in your mug. I actually think this had to do with all the illnesses on board. If they took everyone's mug and touched it up against the soda dispenser, way too many germs would be spread. So, the next time I go, I will not purchase the mug, I would just pay for the sticker.
So, your options are: purchase the sticker or purchase the sticker and pay extra for a mug and make sure you carry your mug if you want to drink out of your mug and not a regular cup.