The U.S. Postal Service owes the Treasury Department $5.5 billion. The bill is due today, but the postal agency cannot make the payment. Nothing will happen in the near term: Post offices will not shut down overnight, and people will still be able to send letters and packages through the mail. But the default underscores not only the financial woes plaguing the nation’s mail system but also dysfunction in a partisan Congress, which controls the Postal Service. Some say privatizing the mail agency is the only way to stanch the flow of red ink. Others blame Congress’ requirement to pre-fund postal retirees’ health benefits. The future of the U.S. Postal Service.