I have been in this exact position myself.
Just remember, the customer has paid extra for you to get a signature from the recipient or at least the occupier of the address, and if you don't, you are scamming THEM.
Just slip leave them, and if the duty holder notices in time which I'm sure he will, then, if you are from another office, just tell him nicely (if you want to keep on his good side and don't want to go all out war) that you know someone who has got in really serious trouble for it before so you don't do it.
It's ok for him, he's only got, say 400 houses to worry about, as a rural duty holder. And I bet he does these favours because he gets tipped at Christmas. But as a floater/reserve, if you start going down this dodgy road, you have potentially thousands and thousands of people who you don't know that could complain about you, and it only takes one to get stung.
Always think to yourself, what need I do to feel clean when I leave for home later? Think only of yourself and your family. If the duty holder gives you grief, it shows he does not care about you. He is using you to try and keep his tip givers sweet.