How do charter schools make money if they're non-profits?

Naturally, no one wants to be paying for a for-profit school as it would then be far too easy to identify excessive charges and then, even if parents didn't make complaints about obvious money-making moves by the school, resentment would grow. The answer is shown in Pro-Publica's piece on profits and schools. Essentially, profits are collected at one remove, with the suppliers of books, furniture, cafeteria food, computers, even teacher training. There is, of course, no competitive bidding for anything as the supply line is set up before the school itself is even built.