Disappointed that International Reply Coupons are no longer a thing too! I used one back in the 1980s to write to the authors of the Power C compiler[1] in the US about a bug (yes, a bug report by mail). I enclosed an IRC in case they wanted to reply. They were kind enough to write back, and didn't use the IRC (but sent it back). They did however include a floppy disk with the fixed compiler, which was nice of them.

[1] Still around: http://mixsoftware.com/product/powerc.htm

I still use them for ham radio QSL cards. They work everywhere that is a member of the Universal Postal Union (nearly all are). You can buy them online from Swiss Post. They will mail them for free.

https://shop.post.ch/en/packing-sending/sending-letters/regi...