They deliver your most fragile parcels. They arrive at your doorstep with the most intimate letters from loved ones. They are U.S. Postal Service workers. But in the Atlanta area, they have also been handling a precious commodity of a different sort: cocaine. Sixteen letter carriers and clerks have been accepting bribes for as low as $250 to transport 2 kilograms or more of the contraband at a time, according to the U.S.