English: During a big chunk of the 20th century and early 21st, members of the "Universal Postal Union" (many of the post offices) would let you, in one country, buy a pre-paid "International reply coupon" that you could, for example, hand or mail your friend in another country. They, in turn, could exchange this coupon for a local stamp good for mailing back to you. These were commonly used by radio amateurs in the US who'd send them to "contacts" they'd make in other countries, so the latter could mail back "QSL" cards (acknowledgements).