Have you ever heard of QSL cards? They have been in use by broadcast stations and radio amateurs for about a century. Briefly they are used by broadcasting stations, especially those on shortwave, to confirm reception reports from listeners at distant locations. Also they are used by licensed radio amateurs (or “hams”) to confirm two-way contacts between them.
You may read more about the subject in the Wikipedia article: QSL card
In addition to sending them using the regular postal system, there is a parallel system for amateur radio QSL cards set up by IARU. In this system many cards are grouped together by country of destination and sent as packets. Some participants in this parallel system also issued their own QSL stamps!
I am an amateur radio operator that regularly sends and receives QSL cards. I am so happy the tradition continues to this day. I also do electronic confirmations, but treasure my physical QSL cards much more!
As a shortwave radio enjoyer, I have collected two cards, one Via WRMI in Florida is Supreme Master TV and the other is from Radio Prague in Czechia, I look to collect more though, and I store them in an separate envelope with other postcards I have gotten
