The U.S. Postal Service said it ended another fiscal year with billions of dollars in losses, an expected announcement that continued to illustrate the agency's shaky financial picture.

The Postal Service said it lost $5.1 billion through Sept. 30 and that the loss would have hit $10.6 billion without recent legislation allowing it to delay a required annual $5.5 billion payment into a fund for future retiree health benefits. But the payment postponed by Congress is now due...