39 U.S.C. § 1205 — Deductions of dues

(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 must 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 that deductions are made, a written assignment which must be irrevocable for a period of not more than one year. (b) Any agreement in effect immediately before 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, must continue in full force and effect and the obligation for that deductions must be assumed by the Postal Service. No that deduction must be made from the pay of any employee except on his written assignment, which must be irrevocable for a period of not more than one year.