The USPS is able to borrow over $15 billion from the US treasury at an extremely low interest rate (1.2%). It is not directly tax payer funded, but tax payers do shoulder the burden when they borrow money. The government also gives them $2.8 BILLION in tax breaks per year.
http://fortune.com/2015/03/27/us-postal-service/
That's nothing, that's the peanuts. This is who's really funding the loses;
https://en.wikipedia.org/wiki/United_States_Postal_Service#Budget
In 2016, the USPS had its fifth straight annual operating loss, in the amount of $5.59 billion, of which $5.8 billion was the accrual of unpaid mandatory retiree health payments
They are stealing the money from the retirement benefits of their own employees.