Yes ... volume discount. That's hardly a new idea. It sometimes works with bananas (especially if they're already a bit over-ripe).
But I find that it's not so popular in the UK as it used to be in the souks
, for instance.
I've also tried the "regular customer" tactic in supermarkets here, but even then the Blighters never want to play ball!
The down side is, of course, that (just like in the shop) you're putting all your eggs in one basket! Which could translate to:- then they'll have you by the [censored], as it were.