To be honest with you, i think the old fella has a point.
Its a very underhanded way of getting someones attention, and yes whilst i agree that the guy has too much time on his hands, and he could be doing better things, i think you will find that he is in fact doing you a favor by bringing it to your attention.
He does have a point, who would trust a firm who has to use tactics of making a simple flyer look like a recorded delivery slip? If id had seen that in my door, id feel equally annoyed to be honest.
It doesnt actually make your business look all that credible, and whilst its a very clever way of getting attention, its giving out the wrong impression. You also have to be careful here, as the royal mail may take a very dim view on your design, and you could open yourself up to all sorts of legal issues.