Meijer already does this, it is just a normal terminal, that knows your order based on your phone or some other scan, which then accepts cash like every coin return machine. It just dispenses change.
Interesting! Do people using cash have a separate checkout flow / exit than everyone else? It seems like it'd be difficult for human security to differentiate between someone that entered the store with their phone (so they can come and go as they please) versus someone who needs to pay at the end before they can leave.