WASHINGTON — The Postal Service sank deeper into debt on Monday after the agency defaulted on a $5.6 billion payment due at the end of September, the second time it has missed a deadline this year to set aside money for its future retiree health benefits.

The agency said it expected net operating losses to be $15 billion for the fiscal year that ended Sept. 30. That loss includes the two missed payments totaling $11.1 billion for the agency’s future retiree funds. This month, the Postal Service also faces a $1.5 billion workers’ compensation insurance payment to the Labor Department. It said on Monday that it would most likely make that payment, but that it would be left with a cash shortage of about $100 million.

Postal Service officials said they expected the shipping of holiday packages and election mailings to help offset some of the losses. Patrick R. Donahoe, the postmaster general, said there would be no disruptions in post office operations. Mail will continue to be delivered on time, and employees and vendors will continue to be paid, he said.

“Customers can be confident in the continued regular operations of the Postal Service,” Mr. Donahoe said.