From the US Postal Service New Link: The mediation process with the National Association of Letter Carriers, AFL-CIO (NALC) has come to an impasse and will now enter into interest arbitration, a process which would conclude later this year. The Postal Service is still in the mediation process with the National Postal Mail Handlers Union,

See the article here:
USPS, NALC Contract Negotiations Heading to Arbitration