The head of the USPS (Postmaster General, currently Trump appointee Louis DeJoy) reports to the Board of Governors [1] (9 governors + PG + Deputy PG) who are nominated by the President; the PG can be removed by an absolute majority of the board. The USPS is overseen by the Office of Inspector General (USPS-OIG), current head Tammy Hull [2] and has a "hotline" (actually a web form) for reporting complaints [3] which fall under its focus areas, which includes fraud, computer crime and employee misconduct. Seems like one place to start.
For previous 2022 discussion of controversies involving Postmaster General DeJoy and what it would take to remove the PG, see [4].
The PG has no term limit but most recent PGs averaged ~5 years. Historically it wasn't seen as a partisan appointment and wasn't replaced when an incoming President changed to the other party.
[0]: https://en.wikipedia.org/wiki/United_States_Post_Office_Depa...
[1]: https://about.usps.com/who/leadership/board-governors/
[3]: https://www.uspsoig.gov/hotline
[4]: "Can Biden fire US Postmaster General Louis DeJoy?" https://www.federaltimes.com/federal-oversight/2022/08/24/ca...