The U.S. Postal Service in September hit its $15 billion borrowing limit from the U.S. Treasury for the first time in its history, leaving the agency with only the revenue it takes in from selling stamps, shipping and other services to cover its operating costs.

The Postal Service has added $2.4 billion to its debt since June 30, pushing the agency to its borrowing cap, said Postal Service spokesman David Partenheimer.

"Being at the limit is a serious situation because our limited liquidity does not give us operating flexibility," he said. "Without passage of comprehensive legislation as part of the Postal Service's business plan to return to financial stability, we continue to project low levels of cash."

The agency hit the limit late last month, though it had sufficient cash reserves to make a $1.4 billion workers-compensation payment on Monday, Mr. Partenheimer said.