Postal Supervisors and Managers Fairness Act of 2021 This bill modifies procedures for negotiating pay and benefits of supervisory and other managerial personnel of the U.S. Postal Service (USPS). At least 60 days before the expiration of a pay decision, the USPS must propose to the supervisors' organization any changes in pay policies, pay schedules, or fringe benefit programs for members of such organization. Not later than 60 days after the USPS agrees to a collective bargaining agreement that affects the pay or benefits of supervisors or managers who are members of the supervisors' organization, the USPS shall provide a proposal regarding such matters to the organization. The bill makes binding the decision of the fact-finding panel appointed by the Federal Mediation and Conciliation Service in the event of an impasse between the USPS and a supervisors' organization over proposed changes in pay and benefits.