Stamped letters were the first casualty of the digital age, made obsolete by the speed of emails. Now, the entire first-class letter system is facing collapse as more and more services -- from banking to bill-paying -- go online.

Faced with billion-dollar losses, the United States Postal Service is considering closing thousands of post offices nationwide and eliminating Saturday service. This week it pegged its latest quarterly loss at US$3.3 billion. Last year it lost $5.1 billion.

The backbone of the mail service -- first-class mail -- is on a steep, steady decline. Experts say they don't see letters making a comeback anytime soon.

According to U.S. Postal Service figures, mail volumes dropped 20 per cent in the last four years.

"First class mail is the bread and butter," Carleton University professor Ian Lee told CTV News. "It is the profit margin of this business. And the business model is falling apart."

Even loyal letter carriers admit their service is growing obsolete. Washington mailman Devlin Johnson said his mailbag has grown progressively lighter over the years.

"I think we'll survive, but it's going to take a little ingenuity," he told CTV.

It's a far cry from when Johnson began delivering the mail 32 years ago. Back then, it was the main way to communicate with someone far away, apart from the phone.

One recent news report quoted Postal Service officials saying it must reduce annual costs by $20 billion by the end of 2015. One plan calls for closing more than 3,500 post offices.

The latest figures have federal lawmakers scrambling to save the 200-year-old institution.

"The bottom line is we must act quickly to prevent a postal service collapse," said U.S. Senator Joseph Lieberman.

With a report from CTV's Washington bureau chief Paul Workman