711 currently have a coupon system where if you buy certain items you get a cash back coupon. At 2 different 711 I have had to ask for the coupons on five occasions in the past week.

Laziness or simply thievery by the staff, they are cash coupons.

Might be because they think you are a tourist (depending on the area) and thus not interested. I always collect those stickers and now the money back coupons from 7-11 (so far have 4 umbrellas, 4 LED torches, a good sized rucksack, a foldup double seater with mini-table and cup holder deck chair, camping table and benches, 2 lilos, various cups/saucers/plates/cutlery sets, 2 Mickey Mouse ice bags, Tupperware kit, and so on...). At first I had to ask, once they knew me they handed them over to me - happened again when I moved a couple of years back (and a new 7-11 was built across the road from me), but always get them now. I thought as you did at first, but once was behind another farang who turned them down, then I was offered mine (which I took) - I asked what they did with his and they said they showed me a poster they stick the "donated" stickers on - the full sheet equals a small payment to a charity. Now it may be of course that some individual are keeping them for personal use, but I think this is generally unlikely to be the case - and that they simply think foreigners are not interested in them so put them on the wall instead.

BTW the tills now always show the points/sticker count/coupons on the customer side which they never used to.

The other common trick, or stupidity, are the special offers:

I've seen packs of Comfort for example at 23baht for 900 ml. Or you buy a pack of 3 - special price 75baht.

Yes I've seen that a lot.

The same with size of tin/packet - in the west a supermarket will have say a half kilo at $1 and 1Kg at $1.90, for example. Thereby to encourage people to buy bigger than they need for the 'bargain' (like Large and Super-Size in McDonalds!) - but here it seems common to have 1/2Kg at 30b and 1Kg at 65b.

I am sure this is working on the fact that Thais generally have very poor mental arithmetic, relying heavily on calculators or paper and pencil; most thereby just grab the biggest if they can afford the money for, and expect it to be the cheapest way to buy.

Number of times I have left with four packs of cat food or cat litter (have 3 cats) from Tesco or BigC rather than a single sack of 4 times the weight - and often the smaller packs also have other incentives like pooper-scoopers or sachet meat, etc.

//Typo: Never left with 4 "backs"