The office I work in is on the second floor of the building. There's a communal letter box on the ground floor by the entrance door.
We were finding, two years back, we were getting lots of "couldn't deliver coz you were out" cards left by the post-office - for two reasons. Firstly, clients posting back bulky tax returns after approval and signature invariably just stuck a 1st class stamp on it and the returns were too large and too bulky to qualify. Secondly, the postman couldn't be assed to climb up the stairs and knock on our door, where we'd happily have paid the excess. (Well, perhaps not happily, but we would have done.)
We challenged the postal authorities on the last point, they were sort of non-commital. Another problem was that if the letter was addressed to the boss, she had to go in person to collect the offending items from the sorting office, and take proof of identity, she couldn't send one of the staff. Not a happy bunny ...
Now, we send the bulk of our letters out by email and scan enclosures. We tell the client to just return (or email back) the signature pages where needed. Since all tax returns - most Government stuff these days in fact - are geared up for on-line filing these days, we no longer post these items to them either.
The loss in revenue to the postal services must be enormous.