It works like this:
1) They send a bill with no information about the services performed.
2a) You waste your time tracking down what it was and then pay it.
OR
2b) You pay the bill without wasting your time to verify.
3) They wait somewhere between a month and 6 months.
4) After receiving payment, they bill you again for the same services.
5a) You waste your time tracking down the services and the fact that you already paid. Then you waste more time calling and forcing them to stop billing you. You get an "Oops, sorry! I can't imagine how that happened."
OR
5b) You pay the bill without wasting your time to verify.
Repeat steps 3 through 5 until they've sucked as much time and money out of you as possible.