All employees of the Federal Government, including Postal Service employees, by law are guaranteed the right to refrain from union membership. See 5 U.S.C. § 7102 (federal employees generally); 39 U.S.C. § 1209(c) (postal employees). (U.S.C. stands for "United States Code.") That means that you cannot be required to remain a member or pay dues or fees to your union, no matter where you work.
Employees of the Federal Government by law are also guaranteed the right to refrain from assisting a union. See 5 U.S.C. § 7102 (federal employees generally); 39 U.S.C. § 1209(c) (postal employees). This means that you cannot be required to pay dues or fees to the union unless you have voluntarily joined the union.
If you are not a member, you are still fully covered by the collective bargaining agreement that was negotiated between your employer and the union, and the union is obligated to represent you. Any benefits that are provided to you by your employer pursuant to the collective bargaining agreement (e.g., wages, seniority, vacations, pensions, health insurance) are not affected by your nonmembership. (If the union offers some "members-only" benefits, you might be excluded from receiving those.)
If you are not a member, you may not be able to participate in union elections or meetings, vote in collective bargaining ratification elections, or participate in other "internal" union activities. However, you cannot be disciplined by the union for anything you do while not a member.
If you are currently a union member, you have the right to resign. If you have authorized payroll deduction of your dues, you would also have to notify your employer that you wish to revoke that authorization. If you signed the authorization less than a year ago, or if it renews automatically each year, revocation may not be effective until the end of the current authorization year. It is advisable to obtain a copy of your payroll authorization and review it.