Read it through once

They have also a small gold coin worth two dollars. And that reminds me of something. When Morocco is in a state of war, Arab couriers carry letters through the country and charge a liberal postage. Every now and then they fall into the hands of marauding bands and get robbed. Therefore, warned by experience, as soon as they have collected two dollars' worth of money they exchange it for one of those little gold pieces, and when robbers come upon them, swallow it. The stratagem was good while it was unsuspected, but after that the marauders simply gave the sagacious United States mail an emetic and sat down to wait.