If I'm buying a few things there are all sorts of places where I'll ask for a discount. If I'm buying two tyres for my car at the same time I'll see if there's a discount available, if I were to have a lot of dental work done I'd look for a discount for immediate settlement or for having all the treatment done at once, if I buy a car I expect something off the screen price.

At the supermarket I know the person behind the till is a minimum wage worker with no authority to vary the prices, and without so much as the authority to let me pay with two £50 notes if my total bill comes to £100.01 - they don't have the freedom to ignore that extra penny. The difference here isn't that I don't want a discount for buying £100 worth at once, it's that I know full well the person I'm dealing with has precisely zero authority to vary the price even a little.

If I'm dealing with the kind of place where the staff have some freedom to negotiate on price a little then by all means I'll ask for a discount if it seems appropriate. So if we can come to a price where I'm happy with the price and the owner is happy that he's not working for nothing then I go ahead and buy. If the owner isn't happy with the profit he won't sell; if I'm not happy with the price I won't buy. It really isn't difficult.