The U.S. Postal Service will default this week on a $5.6 billion congressionally mandated obligation to pre-fund retiree health benefits, marking the second time in two months the cash-strapped agency has done this.

The Postal Service last month failed to pay $5.5 billion for its fiscal 2011 prepayment obligation, which originally was due in September 2011 but was deferred by Congress until Aug. 1. That was the first time it ever defaulted on a payment to the Treasury Department. The $5.6 billion due this week, on Sept. 30, represents this fiscal year’s obligation.