Surely it's happened to you. You've been short changed. Try this, it works all the time,I sent an original of this to the Bangkok Post, but because I named the outlet/organisation, they could not print.
There's one every couple of hundred yards, (no name) pop in on the way home,for a packet of smokes, have a half full Heineken in your hand,and immitate the chime, doo-doo, doo-doo, buy a bottle, optional,as long as you do a good immitation of the chime you should arouse the interest of the perpetrator, point at the cigarette display, and nod with an accompanying, half pissed grin,pass the 1000 bt note over, and wait for the supermarket shuffle!
He/She will bob down ,jump up, rattle a bag of coins, shuffle to the left,bob back down, and produce a bundle of 100 and 50 bt notes, fuck around in the till for 20 seconds, as she hands over the change back to you,the right hand grabs the scanner and serves the next punter, at same
same time.

I guarantee you, your a hundred baht short, 2 hundred if your a real good actor, try it with your friends on the way home from the bar tonight, it's fun, take a photo of the ****, adds to the fun. lol.