WASHINGTON — The U.S. Postal Service, on the brink of default on a second multibillion-dollar payment it can’t afford to pay, is sounding a new cautionary note that having squeezed out all the cost savings within its power, the mail agency’s viability now lies almost entirely with Congress.

In an interview, Postmaster General Patrick Donahoe said the mail agency will be forced to miss the $5.6 billion payment due to the Treasury on Sunday, its second default in as many months. Congress has left Washington until after the November elections, without approving a postal fix.

The Postal Service has been seeking legislation that would allow it to eliminate Saturday mail delivery and reduce its $5 billion annual payment for future retiree health benefits. The post office says it has been seeking to reassure anxious customers that service will not be disrupted, even with cash levels running perilously low.

Donahoe said congressional delays were to blame for much of the losses, expected to reach a record $15 billion this year.

He said the two missed payments totaling $11.1 billion for future retiree health benefits — payments ordered by Congress in 2006 that no other government agency or business is required to make — along with similar expenses make up the bulk of the annual loss. The remainder is nearly $3 billion in losses, he said, which would have been offset if the service had been allowed to move to five-day mail delivery.

Donahoe said the post office will hit a low point in cash next month but avert immediate bankruptcy due to a series of retirement incentives, employee reductions and boosts in productivity among remaining staff that saved nearly $2 billion over the past year.