Public.
The USPS says, "The U.S. Postal Service participates in three retirement plans: the Civil Service Retirement System (CSRS), the Federal Employee Retirement System (FERS), and the Postal Service Retiree Health Benefits Fund (PSRHBF). The first two are pension plans, and the third is set up to prefund and provide retiree health benefits." See this USPS website.
CSRS and FERS are the same plans that civilian employees of the US Government participate in.