>The economy operates most efficiently when things cost what they should or as close as possible.

When things cost as close as possible to their MARGINAL cost. The marginal cost of mailing a letter is very low, almost certainly closer to $0 than $0.83, especially if you accept letter carriers have to run their route every day regardless.

The problem of course is the USPS has enormous fixed costs, including I would argue the pensions (honestly not sure how they would be accounted for in a GAAP sense, I'm absolutely not an accountant)

Federal funding has long distinguished between the two for other things like transit. You're only allowed to use federal funds for capital expenses, not operational expenses. USPS getting a taxpayer subsidy for their fixed costs doesn't seem that nuts.