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 retiremen

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.