39 U.S.C. § 1205

(a) When a labor organization holds exclusive recognition, or when an organization of personnel not subject to collective-bargaining agreements has consultation rights under section 1004 of this title , the Postal Service shall deduct the regular and periodic dues of the organization from the pay of all members of the organization in the unit of recognition if the Post Office Department or the Postal Service has received from each employee, on whose account such deductions are made, a written assignment which shall be irrevocable for a period of not more than one year. (b) Any agreement in effect immediately prior to the date of enactment of the Postal Reorganization Act between the Post Office Department and any organization of postal employees which provides for deduction by the Department of the regular and periodic dues of the organization from the pay of its members, shall continue in full force and effect and the obligation for such deductions shall be assumed by the Postal Service. No such deduction shall be made from the pay of any employee except on his written assignment, which shall be irrevocable for a period of not more than one year.