I don’t do shops, but in general every time a customer argues about the price, I raise it.
Eg:
Me: “the price is £20”
Cust : “Ooh I’ve had cheaper quotes, can’t you do better?”
Me: “yes, the price is now £22”
Cust: “that’s more! I said I’ve had cheaper quotes, surely you can do better”
Me: “it’s now £24, that is better (for me)”
And so on.
Price moaners make the worst customers.