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.