John Rumm formulated on Sunday :
Because they have assumed a certain transaction cost and built it into the
retail price. That likely includes accepting payment in forms which come at a
cost to the trader - such as credit card fees on each transaction. If you
come along and elect to pay in a way which does not incur them some of their
budgeted fees, then they have some more negotiating space. Much the same when
you offer to buy the whole box rather than a one off...

Exactly! I buy a £3,000 laptop for cash, I expect to do a deal. I
expect to start at 20% discount, rather loose a sale, they negotiate. I
buy a car, I negotiate a deal.

I would not expect to go into ASDA and negotiate the cost of a bag of
frozen peas, that would just be silly, but if I spot something on none
food special at Lidl, maybe a box which has been opened/ slightly
damaged - I would discuss the cost with the manager if I wanted the
item and I was happy to accept the damage, providing the cost were
reduced.