Cool little game, I like how overpowered the hammer is.
Just a bit of advice: since walking with the cart is slower, it makes the most sense for the player to leave the cart at checkout and just bring the items straight there in their hands. To enforce the use of the cart and make more of a shopping feeling, you could instead make it so players can't carry items, only the cart, and they roll over items to pick them up, and the cart gets heavier as it's filled up. Then the players are always choosing between loading up the cart a lot or just a little, and will defend it more fiercely.