Hello Kcsc1291--
All staterooms come equipped with a "Beverage Cooler." Basically, it is a low-powered fridge. On our cruises I have always brought 6-8 cans of Coca-Cola to keep in the stateroom and the Beverage Cooler has always done a nice job of keeping them cold.
As far as the Unlimited Soda Package, you may find it to be a good deal or you may not. It really depends on how much soda you will drink while on the cruise. Keeping in mind that sodas will be free in all of the sit-down restaurants at breakfast, lunch, and dinner, you may find that you wouldn't make much use of the mug.
Sodas run just under $2.00 if you buy them individually at bars. That's not a huge cost and is actually quite comparable, or even cheaper, than most restaurants. The soda package runs $35.00 for the sticker. So, you would need to consume at least 3 sodas per day outside of any that were free at the sit-down meals in order to break-even with the program.