About international reply coupons
If you write to somebody in another country and would like them to reply, you can buy an international reply coupon (IRC) and enclose it with your letter. This will entitle the recipient to one or more return stamps corresponding to the minimum postage for a priority item or an unregistered airmail letter to a foreign country.
In short, stamps obtained in exchange for IRCs can be used to reply to the sender.
IRCs cannot be bought directly from the UPU: they are available from local post offices and certain operators. Even if a designated operator of a member country does not sell IRCs, it is still obliged to exchange them.
If a national operator does not sell IRCs, customers can buy them from a post office of a neighbouring country.