The Postal Service posted a record net loss of $15.9 billion in fiscal 2012, much of it due to massive payments the mail agency could not make but still must account for in financial statements.

That is more than triple its $5.1 billion loss last year.

The USPS, which relies on the sale of stamps and other products rather than taxpayer dollars, has been grappling for years with high costs and tumbling mail volumes as consumers communicate more online.

Postal officials have been scrambling to cut costs and boost other parts of the business, such as package delivery, but it has not been enough to staunch the losses. The USPS on Thursday repeated its call for lawmakers to quickly pass legislation overhauling the agency's business model.

"We are still walking the financial tight rope," Postmaster General Patrick Donahoe told the agency's board of governors.

"Our business model is such that we cannot return to profitability without a legislative solution. We simply cannot sustain large financial losses indefinitely," he said.

Earlier this year, the Postal Service defaulted on more than $11 billion in payments that Congress had directed it to pay into a fund for future retiree health benefits. The agency also hit its $15 billion borrowing limit for the first time in 2012.

Excluding those two payments, and another for workers' compensation that the agency did make, the Postal Service would have lost $2.5 billion during the fiscal year, which ended September 30, the USPS said.

Total mail volume tumbled to 159.9 billion pieces in 2012, a 5 percent dip from 168.3 billion pieces a year ago.

While email has obliterated mail volumes, online shopping has proved a boon for the package business as the Postal Service delivers items ordered from e-Bay, Amazon.com and others. The agency said package revenue rose by $926 million, or 8.7 percent, during the year.

Operating revenue fell 0.8 percent to $65.2 billion for the year. Operating expenses, including the two retiree health payments, rose 14.7 percent to $81 billion in 2012.

"The Postal Service is facing a fiscal cliff of its own, and any unanticipated drop in mail volumes could send the agency over the edge," said Art Sackler of the Coalition for a 21st Century Postal Service, which represents business mailers.

"If Congress fails to act, there could be postal slowdowns or shutdowns that would have catastrophic consequences for the 8 million private sector workers whose jobs depend on the mail."

Postal officials want Congress to pass legislation that would allow the agency to end Saturday mail delivery and stop making the retiree health pre-payments, among other things.

Lawmakers have been working for more than a year on legislation to overhaul the Postal Service but have been unable to agree on how to do it.

The U.S. Senate passed a bill in April that would allow the agency to eventually move to five-day mail delivery and spread out the retiree health payments over more years.

Leaders in the House of Representatives have said that bill would only postpone the Postal Service's demise, not solve its problems, but they have been unable to pass a bill of their own.

Lawmakers and observers have said they hope Congress can address postal legislation during the post-election "lame duck" session, but tax and spending issues likely will consume lawmakers' attention during that period.

(Reporting by Emily Stephenson; editing by Andrew Hay)