The deal is only good on small or medium Blasts or Master Blasts and there's a limit of two per order. You have to mention the deal when ordering (it's probably a good idea just to call and ask ahead to make sure your local Sonic is participating anyway).
Sonic Master Blast flavors include Triple Chocolate, Turtle Pecan, Pineapple Upside Down, Banana Split, Caramel Brownie, Waffleberry Brownie, Malted Brownie Bliss, Strawberry Pucker Pie, and Chocolate Chip Cookie Dough.
The regular Blast flavors include Snickers, Oreo, Butterfinger, M&M's, and Reese's Peanut Butter Cup.