The U.S. Postal Service directly employs truck drivers. These individuals carry mail along specific routes and they carry bulk freight. These employees are over-the-road truck drivers who perform duties similar to those of other tractor-trailer drivers. They're considered employees of the USPS, and their pay, retirement, insurance and other benefits are the same as for any other employee of the service.
Under some circumstances, the U.S. Postal Service will hire independent contractors. This contract arrangement works similarly to any other situation in which a company would hire a professional driver for a short-term haul. The driver is paid a negotiated, contracted fee to perform the driving work, and then once that drive is completed their job is effectively done unless there is a more long-term contract worked out.
The U.S. Postal Service may hire a contractor trucking company to take over the delivery of bulk mail. Matheson Trucking is an example of one contractor that has had a standing job with the USPS. This works in a similar way to individual contracting. The USPS negotiates a contract with an entire company, which will then use its own resources to get the job done. The employees work for the contractor company, not of the U.S. Postal Service.