Too bad the tag changing is only the tag, not the actual register database.
This seems like an easy jump. Since the back end system notifies the tag. But also posted price has to be honored in most states. You can’t put $5 on the tag then the register charges 9.99.
So start with tags that cost them money then move forward.
From my understanding, tags are one-way. They receive only which would mean that the "retag" system is probably send-only.
It sure an about commie states, but in non commie owns there are laws that say a published price, even if erroneous has to be honored. Else is false advertising.