I did wonder if you could re-use Painsbury's offer vouchers by feeding the "used voucher" slot with old ones instead. Maybe they have that covered and use the data to TAG you "honest/dodgy". I wouldn't bet against it.
I never get there often enough to try it. But I always feed it out of date ones, and useless ones as soon as they are printed. Simple pleasures - skewing their metrics.

I did read some years ago that supermarkets were trialing e-paper shelf labels. E-paper is like early Kindles, write speed slow but needs no backlight. The only ones I have seen so far were in France.