The price on the shelf and the price the scanner charges are supposed to match, and when they do not, a specific branch of state government is responsible. That oversight is older than barcodes.
Weights and measures is a state function
Every state maintains a weights and measures program, often within its department of agriculture, and much of the routine inspection is delegated to county or city sealers.
The office's traditional work is verifying scales, fuel dispensers and package contents, which is why the certification sticker on a supermarket scale carries a state seal and an inspection date.
Scanner accuracy was folded into the same program as barcode systems spread, because a scanner that charges the wrong price has the same effect as a scale that weighs short.
Inspections use a defined sampling method
An inspector selects a sample of items from the sales floor, records the advertised or shelf price, then takes them through a register and compares the result.
National model procedures set the sample size and the tolerance, with a common benchmark requiring accuracy at or near a very high threshold and treating overcharges more seriously than undercharges.
Failing a check can trigger reinspection, fines or an order to stop selling by scanner until the price file is corrected, depending on the state's statute.
Errors usually come from the price file
Scanners read a code and look up a price in the store's system, so a mismatch almost always means the shelf tag and the file disagree rather than that the scanner misread.
Promotions are the most common source, because a sale price has to be loaded centrally, printed on a tag and started on the right day across many stores at once.
Multi-buy offers and loyalty pricing add complexity, since the discount may apply at the end of the transaction rather than at the line item, which looks like an overcharge mid-scan.
Remedies vary by state and by store
A few states require a specific remedy for a scanning overcharge, such as providing the item free up to a value limit or refunding a multiple of the difference.
Elsewhere the remedy is store policy, and many chains adopt a similar approach voluntarily because it is cheaper than an inspection complaint and it defuses a checkout dispute quickly.
Because the rules differ, a shopper who wants to know their rights should check their own state's weights and measures office rather than rely on a policy seen in another state.
Complaints feed the inspection schedule
Inspectors work on a routine cycle, but consumer complaints direct additional visits, and a store generating repeat complaints attracts closer attention.
Reports are typically made to the county sealer or the state office, and they need the store, the date and the specific item to be actionable.
That reporting channel is the reason the system works at all, since no inspection program has the staff to check every price in every store on its own.