If they refuse to check you out unless you put on a mask, just say no thanks and leave the cart full of food right there. They will have to devote people to put stuff away quickly (they have time requirements for cold or frozen stuff). If enough people do this the grocery stores will give up.

I never masked in stores. Only place I ever half-wore them was in hospitals when they were crazy mandatory to even come in then I'd take them off or pull it down.