They include some of the heaviest hitters OpenSecrets.org keeps track of, like postal unions and FedEx, as well as groups from more obscure corners of the lobbying world, like the Envelope Manufacturers of America (yes, envelopes have lobbyists)
Of course, lawmakers have heard from FedEx and UPS, both of whom have a lot riding on the viability of their biggest competitor. Both also qualify as heavy hitters on OpenSecrets.org for their intense Washington presence on the lobbying and campaign finance scene (both companies even own townhouses near the United States Capitol building to host fundraising events for lawmakers).
And, with tens of thousands of public employees affected, the unions and groups that represent those workers have been out in force: there's the National Association of Letter Carriers, and the National Postal Mail Handlers Union, both of which are affiliates of union heavyweight AFL-CIO.
There's also the National Rural Letter Carriers Association, which represents mail-carriers far from the big metropolises. The National Association of Postmasters goes to bat for managers, and the National Star Route Mail Contractor Association backs contractors the post office uses to ship mail. The National Active and Retired Federal Employees Association had to say something about this, since most suggestions involve changes to the pension fund for postal employees.
And those are just the heaviest of the heavy hitters in the fight. Check out the full spectrum of groups that have lobbied on the two pieces of legislation, including some surprising names.