A shop/business that hasn't figured out how to address this issue, will soon run into a cash flow problem.
In the past in the days of non-computerized sales receipts it would be possible to convince a young store employee to refund the higher priced one. But with the modern POS systems, the computer should quickly grasp the correct amount to refund.
You can frequently see the approach they will take when reviewing the receipt. If the price is $100 for each, but 3 for $240; the receipt will show the first one for $100, the second one for $100, and the third one for $40. The computer will only let your return the one for $40 if you only bring one back.
There is a department store near us that has many coupons and discounts, which if you buy an item can sometimes trigger more coupons and discounts. If you return an item it correctly un-applies all the coupons and discounts so as to not create an opportunity to game the system.
You can even see this happen during the transaction. At my grocery store they sometimes have an electronic coupon that get your a $10 discount if your purchases in a single transaction is over a certain amount. The amount varies, it sometimes is very easy to meet at $50, other times it is $100 or $150. when the cashier scans the last item and hits a key, the first discount applied is the $10 discount if you are over the threshold. But then the other discounts and coupons are applied, if you fall below the threshold the $10 is then added back into transaction, and is displayed in a red font.