Costco doesn’t sell 2-liters or 12-packs of soda; instead, they sell packs with 35 or 36 cans. If you don’t go through soda like a fiend, it's probably not the best choice.
Over time, even soda in sealed cans will begin to lose its carbonation and flavor quality. Moreover, a case will run you around $20 to $22, which is a lot to spend.