39 U.S.C. § 2010: Restrictions on agreements

The Postal Service shall promote modern and efficient operations and should refrain from expending any funds, engaging in any practice, or entering into any agreement or contract, other than an agreement or contract under chapter 12 of this title, which restricts the use of new equipment or devices which may reduce the cost or improve the quality of postal services, except where such restriction is necessary to insure safe and healthful employment conditions.