WASHINGTON (AP) — The U.S. Postal Service is bracing for a first-ever default on billions in payments due to the Treasury, 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 perilously low, two legally required payments for future postal retirees' health benefits — $5.5 billion due Wednesday, and another $5.6 billion due in September — will be left unpaid, the mail agency said Monday. Postal officials said they also are studying whether they may need to delay other obligations. In the coming months, a $1.5 billion payment is due to the Labor Department for workers compensation, which for now it expects to make, as well as millions in interest payments to the Treasury.

The defaults won't stir any kind of catastrophe in day-to-day mail service. Post offices will stay open, mail trucks will run, employees will get paid, current retirees will get health benefits.