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

The post office 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 post office also faces a $1.5 billion workers’ compensation insurance payment to the Labor Department. The post office said Monday that it would most likely make that payment, but that it would leave the agency 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.

Despite the losses, Patrick 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,” Donahoe said.

The post office had warned Congress for months that it would not be able to make the payments into the fund for its future retiree health benefits.

The payments are required by a 2006 law and do not affect current retiree benefits.

Lawmakers left Washington last month without passing legislation that would have helped the post office deal with its crippling debt and its operating losses.

The Senate passed a postal bill that would give the agency some of the changes it seeks. The House has not passed its version of the legislation.

Although Donahoe said he expected Congress to take up the measure when it returns after the elections, passage remains uncertain. Lawmakers will have to devote much of their time during the lame-duck session to dealing with the “fiscal cliff” – the end-of-the-year deadline for the expiration of hundreds of billions of dollars in tax cuts and for billions in across-the-board spending cuts.

For now, the agency said it was doing what it could to lower costs, like reducing staffing levels and closing mail processing facilities.