39 U.S.C. § 403 — General duties

(a) The Postal Service must plan, develop, promote, and provide adequate and efficient mail service at fair and reasonable rates and fees. It must receive, send, and deliver letters, printed materials, and packages across the United States and its territories and possessions, and — through arrangements made under sections 406 and 411 of this title — throughout the world. It can offer other related services that it decides fit its functions and the public interest. It must try to serve, as much as practical, everyone in the country. (b) The Postal Service is responsible for: (1) running an efficient nationwide system to collect, sort, and deliver mail; (2) offering different types of mail service to fit the needs of different kinds of mail and mail users; and (3) building and locating post offices so that, while still keeping operations reasonably economical, people across the country have ready access to essential postal services. (c) When the Postal Service sets rates, fees, and classifications, or provides services under this title, it cannot unfairly or unreasonably discriminate among mail users, and it cannot give any mail user an unfair or unreasonable advantage — except where this title specifically allows it.