"When buying groceries—food and non-alcoholic beverages, pet food or supplies, disposable paper or plastic products, soap, household cleaners, laundry products, or light bulbs—you must be charged the lowest displayed price, whether on the sticker, scanner, website, or app.
If the lowest price you saw for an item is $10 or less, and that lowest price is not what you were charged or not what appeared on the in-aisle price scanner, the first item should be FREE. If the lowest price you saw for an item is more than $10, and that lowest price is not what you were charged or not what appeared on the in-aisle price scanner, you should receive $10.00 off the first item."
https://www.mass.gov/info-details/consumer-pricing-accuracy-...
Not to say it's not happening in a Mass based Dollar Stores but you could be walking away with a lot of free stuff and it would be enough of a deterrent to stomp out the practice. I've had it happen at grocery stores usually at their suggesting.