39 U.S.C. § 403 : US Code - Section 403: General duties

      (a) 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.
      (b) It shall be the responsibility of the Postal Service - 
        (1) to maintain an efficient system of collection, sorting, and
      delivery of the mail nationwide;
        (2) to provide types of mail service to meet the needs of
      different categories of mail and mail users; and
        (3) 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.

      (c) 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.