WASHINGTON — The Postal Service reported a record $15.9 billion net loss Thursday for the fiscal year that ended Sept. 30, bringing the financially troubled agency another step closer to insolvency.

The expected loss, more than triple the service’s loss last year, included accounting expenses of $11.1 billion related to two payments that the agency was supposed to make into a future retiree health benefits fund. But because of revenue losses, the agencywas for the first time forced to default on these payments, which were due in August and October.