You don't pay for how much the item in the store costs, you pay how much it will cost to restock it (see also: petrol/gasoline prices changing throughout the we

You don't pay for how much the item in the store costs, you pay how much it will cost to restock it (see also: petrol/gasoline prices changing throughout the week - it's because there's a fixed charge to refill the tanks, regardless of how much goes in).