I mean, I'm not an expert, so I might be misreading here, but section 803 of your second link seems to say essentially that the USPS must pay into the retirement fund pre-emptively.

"Requires the Postal Service, beginning in 2007, to compute the net present value of the future payments required...Directs the Postal Service, for each year, to pay into the above Fund such net present value and the annual installment due under the amortization schedule"

The "50 years" part that you bring up is probably an exaggeration to some extent of the amount of time between when a person starts working for the USPS and when they retire, but the general concept behind the argument remains the same I would think.

I agree there is room for private businesses to operate under the umbrella of regulation.

However, the USPS is designed to operate in circumstances where huge losses are inevitable (like shipping to remote towns always at a loss). We already have for profit shipping companies so I see no problem with the USPS, and actually applaud my tax dollars going towards giving every American access to mail.