Do you know IRC?

The IRC(International reply coupon) was introduced in 1906 at a Universal Postal Union congress in Rome. At the time an IRC could be exchanged for a single-rate, ordinary postage stamp for surface delivery to a foreign country, as this was before the introduction of airmail services. An IRC is exchangeable in a UPU member country for the minimum postage of a priority or unregistered airmail letter to a foreign country.The current IRC, available since July 1, 2009, is called the "Nairobi" model, and is available from post offices in more than 70 countries. IRCs are ordered from the UPU headquarters in Bern, Switzerland, by postal authorities. They are generally available at large post offices; in the U.S., they are requisitioned along with regular domestic stamps by any post office that has sufficient demand for them.

A British 1 Shilling IRC issued in 1959.
Prices for IRCs vary by country. In the United States in September 2011, the purchase price was $2.10[1]USD. IRCs purchased in foreign countries could be used in United States toward the purchase of postage stamps and embossed stamped envelopes at the current one-ounce First Class International rate ($0.98 USD as of January 2, 2011) per coupon.[2]
IRCs are often used by amateur radio operators sending QSL cards to each other; it has traditionally been considered good practice and common courtesy to include an IRC when writing to a foreign operator and expecting a reply by mail.[3]
Previous editions of the IRC, the "Beijing" model and all subsequent versions, bear an expiration date. Consequently, a new IRC will be issued every three years. The current IRC will become obsolete on 31 December 2013. Current stockpiles in the hands of users should be expended by then, or exchanged for the new issue to be introduced at that time. The current issue IRC may be exchanged until 31 December 2013 (date printed on coupon).