Dear Postman,
Do you know how and when to use a doorbell?
I know there's a knocker on my front door but you used it rather softly; it's actually quieter than the letter box, to which we attend at our leisure. I was upstairs in the bathroom and David was asleep when you came and thought there was post but no knocking.
As a result of your failure to use the doorbell, the foreign currency I ordered for David's trip abroad will not now arrive until after he has left, meaning we will have wasted time, trouble and money. David will now have to get cash elsewhere, at less favourable rates.
Thanks a bunch!