Let's run the numbers and see if it works the way you say: In 2005, the last year used by the 2006 congress law, the unfunded liabilities of USPS were determined as between $50 and 59 billion [1]. In 2006, congress required USPS to make 10 payments of $5.5 billion. This doesn't meet your characterization os a 1/30th of NPV each year! It is basically paying to whole NPV in a staggered 10 years pre-payment plan.

1: https://about.usps.com/who-we-are/financials/annual-reports/... (page 26)