39 U.S.C. § 403: General duties

The Postal Service shall plan, develop, promote, and provide adequate and efficient postal services at fair and reasonable rates and fees. The Postal Service shall receive, transmit, and deliver throughout the United States, its territories and possessions, and, pursuant to arrangements entered into under sections 406 and 411 of this title, throughout the world, written and printed matter, parcels, and like materials and provide such other services incidental thereto as it finds appropriate to its functions and in the public interest. The Postal Service shall serve as nearly as practicable the entire population of the United States. to maintain an efficient system of collection, sorting, and delivery of the mail nationwide; to provide types of mail service to meet the needs of different categories of mail and mail users; and to establish and maintain postal facilities of such character and in such locations, that postal patrons throughout the Nation will, consistent with reasonable economies of postal operations, have ready access to essential postal services. In providing services and in establishing classifications, rates, and fees under this title, the Postal Service shall not, except as specifically authorized in this title, make any undue or unreasonable discrimination among users of the mails, nor shall it grant any undue or unreasonable preferences to any such user.