You’d buy coins from the U.S. government at face value, pay by credit card, and get free shipping. There were ever-changing rules about how many coins you could buy and how often, but for a time it was unlimited – and some people bought seven figures worth.
The coins were heavy. You had to take them to the bank. And you had to track shipments. But that there was no net cash cost to the deal, and no real limit for awhile on how much you could earn, plus that it lasted in one form or another for about three years makes it an all-time classic.