While it's possibly not the practice everywhere in the US, at a couple of Postal Stations I frequent, during "busy times" there usually are two or three agents for general postal business and at least one other at a separate "window" marked "pickups only" for those who only want to get mail from "general delivery" or who have received notice that something couldn't be delivered.

Attempting to go to the "pickup" window to send something when the place is crowded might cause objections from others, and if the station is not marked it could be just "tradition" that reserves that line(?), assuming there's another line for other stuff. Or again custom may dictate that there's a particular time when "the check is in the mail."

The assumption that "everybody knows" is not the exclusive province of the elderly. And "everybody knows" that you should never have to explain what "everybody knows" to anyone who isn't "in the know."

John