This is fascinating. We have no such system in the US as far as I am aware. First, I think it’s against the law for anyone other than postal employees to put stuff in mailboxes. So we get flyers shoved under doors, in cracks around letterboxes, etc. Second, no one would respect those stickers. And third, because our postal service has to be self-sustaining and legit mail has declined, the only mail you get these days is advertisements.