I actually had a job for a couple of years designing 'advanced' POS system software based on the industry standards. The items that the retailers considered 'advanced' just amazed me. They seemed like basic items to me. That said, it is almost impossible to protect against this until they changed the coupon codes to something like QR codes that they could digitally sign. The current standard was made long time ago and is very limited in the amount of information it can carry.
This is a simple standard anyone can google that shows exactly how these bar codes are laid out. I used that myself when implementing that section of code. Why anyone would pay for it is beyond me. At the end of the day, the retailers know there is going to be some fraud on this. They have switched to all of these automated checkout systems to save money on cashiers, who would likely catch this sort of thing, so I don't really feel sorry for them at all. They have made that trade.
That said, there are some safeguards in there. If your coupon total is unusually high, your checkout will lock up and require approval from the person watching up front. So anything outrageous will likely be caught. Also, you better pay with cash, as the transaction is all tied together to your card. Also, don't use a loyalty program. So at the end of the day is all that hassle worth it to maybe save a couple of dollars? Conversely, though, I guess the checkers care less and less every day as they get replaced by machines and probably just hit the 'ok' button most of the time, and the retailers eat it as just cost of doing business.
Myself, I would not suggest it as it is punishable by law. It is worth going to jail to save a relative few dollars off your bill?