>The basic idea is that consumers overpay by sending a UTXO for the product in its entirety, and the retailer returns the change

I must be misunderstanding what is meant by "overpay" and the use of the word "change".

If the expected behavior for users of LN is to esclusively pay in what os the cash equivalent of large denominated bills, i.e a 100$ note for a 75$ purchase, and exact change held by a consumers somehow makes the system less efficient, we are in for a bad time.