International Reply Coupon
An IRC was part of an international postal system that allowed correspondents from one country to pay the postage of someone replying from another country. For instance, somebody writing from Italy to the USA could pay the cost of postage stamps in their own country and send a coupon which could be redeemed for the equivalent cost in America.
As the cost of postage was significantly lower in Italy than in the U.S., Ponzi saw an opportunity for a profit to be made by trading the coupons. Based on the current difference in rates, there was a potential to make around 400% profit on every coupon. Moreover, it was completely legal.
