WASHINGTON — The Postal Service, on the verge of its first default on Wednesday, faces a cash shortage of $100 million this October stemming from declining mail volume that could balloon to $1.2 billion next year, newly available documents show.

Confronting $11.1 billion in payments over the next two months for future benefits, the service said it would fail to pay about half that amount, which is due Wednesday, and does not foresee making the other half, which is due in September. An additional $5.6 billion payment due next year is also in question.

The service is struggling for ways to cut costs, but it cannot eliminate Saturday delivery, as it wants to, without Congressional approval, nor can it slow delivery of the mail without regulatory approval.

The Postal Service had hoped that Congress would help stanch the losses, as it did last year when it deferred the payment that is due again on Wednesday. But the House has taken no action. The Senate passed a measure that provided incentives to retire about 100,000 postal workers, or 18 percent of its employees, and allowed the post office to recoup more than $11 billion it overpaid into an employee pension fund. The Senate declined to act to stop Saturday deliveries.