USPS was completely self-sustaining until 2006, when Congress inexplicably began requiring it to pre-fund retirement health benefits for all of its eligible employees and retirees, 75 years in advance. This unprecedented requirement cost USPS more than $5 billion per year for 16 years … which is almost exactly the amount they were in debt.
And still, USPS maintains First Class mail delivery to every household in the country within 5 days for less than a dollar. No private carrier comes close to their price point (which USPS actually doesn’t control), and many of them actually use USPS as their “last mile” carrier in rural areas where it’s too costly for them to operate.