Are you sure this is the law? You advertise something for a price, the item is paid for, invoiced and delivered. At what point does such a law kick in? If I sel

Are you sure this is the law? You advertise something for a price, the item is paid for, invoiced and delivered. At what point does such a law kick in? If I sell something 5% can I claim it was a mistake? What about 25%, 50%? Where is that line?