It's right there in the name -- and it's right there in the Constitution, which authorizes Congress to establish post offices without a word about turning a profit. [...]
Few people think of the National Park Service or the United States military as bureaucracies that "lose" money, even though they cost the government billions of dollars. So why should the Postal Service be treated any differently?
By design, the Postal Service was destined to spend more money than it makes. This was arguably best articulated in the Postal Policy Act of 1958, which stated that the post office is "clearly not a business enterprise conducted for profit." To the contrary, Congress noted, mail delivery is a public service that promotes "social, cultural, intellectual, and commercial intercourse among the people of the United States." Indeed, the post office was thought of by the Founders as a means to connect Americans to each other at little cost, and to ensure that the public is well informed -- a critical need for any democracy, especially a young one as the United States was at the time -- by subsidizing the delivery of newspapers.
For most of its existence, the post office was a federal department and the postmaster general was a member of the president's cabinet. As a result, Congress funded it just as it did all other federal departments. That changed in 1970, when Congress, under Richard Nixon, passed the Postal Reorganization Act, which turned it into an independent federal agency that was required to cover most of its costs, with little help from Congress. Since then, the rhetoric of "profits" and "losses" at the post office took hold.