When I was working minimum wage at a gas station many eons ago, we would have '2 for $x' specials where x is less than 2 times the individual price of whatever item.
People would often not want to buy 2, but I would ring up 2 in the till for the special price and charge them for the single. Then when the next person did the same, I would charge them for the other single.
So over the day, I would sell 10 energy drinks at say $4, but ring them up as 5 '2 for $6' specials. This would put the till up by $10, and then I would use that $10 to have a free meal.
Someone I worked with did something similar. This was close to 25 years ago, at a popular drive thru corporate coffee chain. I took no part in these shenanigans myself, but I got a cut each night we worked together just for looking away as it happened.
Basically, her whole hustle revolved around the fact that the cash register at the drive thru window had been broken and not working for ages. She had memorized the prices for every single item sold there. So when someone ordered at the drive thru, she would tally up their whole order in her head, but then go around to the main counter's register and ring the entire order up as just a small coffee. She'd then take their money, give them the change that was due, but put the difference into the tip cup.
This went on for about a month until I just couldn't stand the overall work environment. It's still to this day the only service industry "job" I've ever had. I hated it so much that even all that extra free money couldn't keep me there.
Hating that one month so much is also the reason I tip service workers as best I can, still to this day.