I’ll go with: 2) The teenager didn’t cheat, exactly, but he “shaded the letter of the law” (or custom, in this case)

I imagine no one would have noticed or cared if it was part of the father’s transaction. Or, if the teenager had been on their own, unloaded their own full basket, realized they forgot milk or eggs, ran to get some while the cashier began ringing (although many grocery stores are happy to send someone to grab it for you).

The dad should have just thrown them on his transaction and either covered it or had the teenager give him cash. But I wouldn’t have been bothered by it.