What lies ahead for the U.S. Postal Service if nothing is done to brake its financial slide?  Postmaster General Pat Donahoe tossed out an attention-getting benchmark today. “This is Greece,” Donahoe told participants at the PostalVision 2020 conference, as he highlighted a slide showing the Postal Service with $92 billion in debt by 2016 (up from about $12 billion last year), assuming that Congress doesn’t act on pleas for relief from the requirement to pump billions of dollars annually into a health care fund for future retirees. Greece, he said, has a debt-to-gross domestic product ratio of 1.6 to one.…