How is the United States Postal Service going broke when they are government run? I thought the government can't go broke?

I never understood how the Postal Service has money problems when they are part of the government umbrella. Isn’t by nature the US government the only entity that is legally able to go into debt without worrying about going broke. I hear about all these layoffs with postal workers but am confused because if they are government run, how in the world do they have to make budget cuts in this manner.