ThinkProgress put it this way:
"Under the Postal Accountability and Enhancement Act, Congress has for years forced the USPS to pre-fund 75 years’ worth of pensions for its employees, a requirement not made of any other public or private institution. That means that the Postal Service is footing the bill for employees it hasn’t even hired yet.
The USPS doesn’t actually receive money from the government, but still needs Congressional approval to make any changes to its structure. An analysis in July showed that the USPS, without its pension requirement, would have a $1.5 billion surplus."
We need to urge Congress to repeal this as soon as possible.