Then you're using a different definition. In this context, self-sufficiency refers to its financial and organizational position within the United States government as an agency to manage its own finances externally from the Congressional Budget Office and has nothing to do with its bottom line. The USPS is currently running a deficit but has also ran a surplus:

"Despite record gross revenue of $60.1 billion, the U.S. Postal Service ended fiscal 1998 on Sept. 30 with a $600 million profit, $100 million more than projected, but leading industry officials were hoping the USPS could at least match, if not better fiscal 1997's $1.2 billion surplus, which was less than the $1.6 billion recorded in 1996 and significantly lower than the $1.8 billion achieved in 1995." http://directmag.com/news/marketing_observers_fear_end/

Corporations borrow money, especially when they run a deficit. Yet unless they're a larger part of some organization or the government, most people would call them self-sufficient--the ability to manage their own financial resources.

Sure the most strict definition of "self-sufficient" is to have absolutely no reliance on any external entity for anything, but by that definition then not a single organization in this country is self-sufficient.