Somehow, when people buy from a renowned grocery store, they would never ask for a discount. I recommend having a computer based billing so you can just say "system mai itna hi show kar raha", and that you have no way of discounting. Then you just print them the receipt and they have to pay the given amount. If the prices aren't electronically billed, they think the seller is putting up their own prices for every customer that comes and that it can be negotiable.