My understanding is, unlike any other government agency or business, the USPS is required by law to fully fund 50 years of pensions.
If that requirement were removed, it would be fine.
This is an anecdote not data, but I've heard several USPS delivery folks mention that a lot of long-tenured drivers decided to take vacation and sick time when the lockdown started ramping up. This has caused other drivers to pick up the slack so there may be a lot of overtime getting paid out.
I can't get the numbers to add up here. Even with letters down, advertising down then their revenue should either be flat or have a slight increase based on my guesstimate for their increase in parcel business.