This is true, but they have been contributing to that retirement money. The Postal service was in the black, until congress decided they must prefund 75 years in 7 years. On top of that, the Postal service had escrow accounts that congress stole, and spent at the same time. If you look at the OMB site, one of their historical do ents shows the postal service money flow. Congress would cry when they were in the red, but gladly spend the money when they had excess revenue.
I believe the retirement prefunding requirement is the sole cause of the Postal service being in the red. Worse yet, there is no fund. Congress just spends it.