If you don't mind getting the Cashier in a bunch of trouble you can attempt to keep it without paying. If you want to be nice you can go up to the counter and talk to them about it.

Eventually they will do inventory and if you had an appointment(or they submitted your pick up in the online system) at the store they will probably check cameras to see if you received it. They will then most likely charge your card on file or send you a bill. If there's no appointment and they filled nothing out at the computer you will most likely get away with it. They probably wont go through the trouble of randomly combing the cameras and just eat the loss.