I send $3 U.S. with QSL (ham radio) cards. It seems like everybody is able to convert that to local currency to cover postage.

I send international reply coupons with QSL cards. They no longer sell them in Ireland, where I live, but you can still order them from Swiss Post, online. They work everywhere. They mail them to you for free.

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