I believe Michigan has laws on the books that should be the model for this (the "Scanner law") - if you're overcharged at the register and the sale is completed, you have 30 days to get the price corrected plus ten times the amount of overcharge (between $1 and $5). Paying you the 'bonus' is optional, but if they don't do so you can file a suit for the greater of your actual damages or $250 (in small claims on your own or regular court which allows up to $300 in attorney fees).

An alternative would be to force stores with mischarge rates exceeding a specified level to close until they've completed a full audit of all shelf prices in the store but in some areas that could cause significant local hardship.