How can this be hard? There are official USPS mail drop off boxes in fixed locations. They rarely move. They know where these are because they pick up from them every day. The location of the nearest one should not be lore handed down from generation to generation, it should be a searchable database.
The USPS has all its post offices in a locator, but not the boxes. Google Local has all the private companies from the phone listings that do mail stuff, but not the boxes. But everyone needs to know where the mail boxes are, not least businesses in the area. Every place I've worked there has been some mail every day that someone drops off in a mail box on the way home.
I am confused.