WASHINGTON – The U.S. Postal Service is bracing for a first-ever default on billions in payments due to the Treasury. That's adding to widening uncertainty about the mail agency's solvency as first-class letters plummet and Congress deadlocks on ways to stem the red ink.

With cash running low, the Postal Service says it will not make two legally required payments for future retiree health benefits — $5.5 billion due Wednesday, and another $5.6 billion due in September.

The defaults won't stir any kind of short-term catastrophe — post offices will stay open, mail trucks will run, employees will get paid.