Postal service clerks, also known as window clerks, sell stamps, money orders, and mailing envelopes and boxes. They weigh packages to determine postage and check that packages conform to acceptable conditions for mailing. They register, certify, and insure mail and answer customer inquiries pertaining to postage rates, post office boxes, mailing restrictions, and other issues. They help customers fill out forms for damaged packages.

The mail sorters, processors, and processing machine operators prep incoming mail for outgoing distribution. These workers are called mail handlers, distribution clerks, mail processors, or mail-processing clerks. They load and unload postal trucks and move mail around the mail-processing center with forklifts and hand trucks. They load and operate mail-processing, sorting, and stamp-canceling machinery.

The USPS employed 75,000 clerks, 335,000 mail carriers, and 209,000 mail sorters, processors, and processing-machine operators in 2004.

Even though technology has advanced to make the process of delivering the mail easier, mail sorters, processors, and processing-machine operators still engage in physically laborious work. They must move heavy bags of mail throughout the mail-processing center. There is a lot of bending, lifting, and lugging. It can be tedious and tiring work. Many sorters, processors, and machine operators in large post offices often work at night or on weekends because the bigger facilities are open around the clock.