Geo, you're right that the gov't doesn't divert money from the USPS (at least I see no evidence of that), but you're omitting something important - the real cause of their insolvency, as stated in your link, and it IS Congress's fault:

"Fredric Rolando, president of the National Association of Letter Carriers, notes that the onerous health payment for future retirees -- something not required of any other government agency or private business -- is to blame for much of the post office's red ink. He faults Congress for mandating the payments in 2006, saying they force the post office every year into a "panic mode that absorbs energy and resources" rather than focusing on longer-term innovation."The word 'default' sounds ominous, but in reality this is a default on the part of Congress," Rolando said.In 2007 and 2008, the Postal Service initially had profits of roughly $3 billion but fell into the red after making the health payments. In more recent years, it has suffered annual losses of $2 billion to $5 billion even after factoring out the health payments; by 2016, the mail agency expects to lose $21.3 billion a year, of which $5.8 billion will be caused by that payment."

This is because of the Postal Accountability and Enhancement Act of 2006 (PAEA), which obligates the USPS to prefund 75 years' worth of future health care benefit payments to retirees within a ten-year time span – a requirement to which no other government organization or private business is subject. THAT is what is breaking the back of the USPS.