I have sailed on the Dawn last year and the Sea 2 years ago. Both offered soda cards, price was around $29 for a seven day cruise. When you buy it, you'll probably get a container of some sort (for the soda) and a sticker for your room key. Although they may say you need the container to get refills, you really just have to show the sticker on your room key. If they ask, tell them you forgot the container in your cabin, and they'll serve you a refill in a plastic glass. Hope this helps.
I know there are alternate views out there, but my wife and I drank soda because we found the tea to be an instant mix, and not up to par. We got the cards and two cool little coke glasses that we still have and still use. We went on the Star in June of last year.
If you go on one of the Hawaii trips the soda card is a lifesaver on Fanning Island where the price of soda escalates once you get there. I don't drink alcohol so the soda card was a terrific bargain for me.
If you're talking about your room key, it's plastic, like a credit card. The "soda card" is not really a card at all; they give you a sticker that you put on the back of your room key. That way you always have it with you, and you just show your room key to get your soda free.