Reporting a $3.1 billion loss for its third quarter, the U.S. Postal Service today warned that without congressional changes it would default next month on a $5.5 billion payment to the federal government.

For the nine months that ended June 30, the Postal Service lost a total of $5.7 billion. Mail volume continued to decline, led by first-class services, as more consumers relied on e-mail and electronic bill-paying. Total volume fell to 39.8 billion pieces, a drop of 2.6% from the same period a year ago.

Though regulated by Congress, the USPS does not receive taxpayer funding. The default threat has raised the specter of a bailout.

"We are experiencing a severe cash crisis and are unable to continue to maintain the aggressive prepayment schedule," Joseph Corbett, the agency's chief financial officer, said in a statement, Reuters reports. "Without changes in the law, the Postal Service will be unable to make the $5.5 billion mandated prepayment due in September."

The payment is for projected health care costs for future retirees. In 2006, Congress ordered the USPS to pre-fund projected health care benefits for future retirees for 10 years, at a cost of about $5 billion a year. No other business faces such a mandate.

Congress is considering four major proposals to rework the Postal Service's finances. The Hill explains them. One bill, co-sponsored by House Oversight Committee Chairman Darrell Issa (R-Calif.), has attracted Tea Party support, the American Postal Workers Union says.

Last month, the Postal Service announced it is considering closing 3,700 post offices and ending Saturday service next year.

Here's the USPS news release. If you dare, read the 10-Q financial report.