I don’t see it as a problem. Many fast food places (Schnitzi, pizza stores, ect) have a line for food, you progress to the register, and the fridge is off to the side, and self service.
You had a bag in hand, so its stands to reason that you paid for what you ordered at the cashier, and I see no reason to suspect that you would steal the soda.
Plus, you look honest, so that would nail it for me!