A bit confusing until deciphering the Britspeak, where “letterbox” means both an actual box (whether with a slot or lid) and a slot through a door. The article

A bit confusing until deciphering the Britspeak, where “letterbox” means both an actual box (whether with a slot or lid) and a slot through a door. The article refers to things that fit through a standard mail slot. See also https://en.m.wikipedia.org/wiki/Letter_box https://en.m.wikipedia.org/wiki/Letter_box