Where in the law the USPS is required to future pay pensions of 75 years?
Sec 8909a(d)(3)(A) defines the amortization table of advance payments, and the enclosing section defines further payments beyond FY 2017.
PS The 75 years comes from the OPM requirement that government agencies us that number of years for accounting purposes for figuring furture liabilities. It is used by the DoD, Social Security, etc.
You're asserting that the OPM requires all federal services to, essentially, fund in advance the retirement plans for workers that have not yet been born. How about a link to the law defining this, or an article from a credible site?