Most point of sale terminals only verify that the data is the correct format; it does NOT validate or authenticate. The cashier should act as a second check (theoretically a barcode printed on toilet paper for 100% off a TV would work for instance). The coupon gets collected and sent to a 3rd party clearinghouse 95% of the time. That clearinghouse will check for fraud and authenticity, and invoice the manufacturer on behalf of the retailer at the end of a month.
Source: Led marketing for multiple CPG/Food companies, lmk if other questions, I don’t often get to talk about this on here!