They breached the contract, so yes, you can take legal action. But what I would weigh before doing so is what it would cost you to take that legal action, versus that they didn't give you thirty days written notice. While I think what they did was a jerk move, it may not be worth it to pursue it. And maybe their new biller will stink, and they will want you back...I say leave it alone, and if they do come back to you, forget the percentage billing, and charge them $15 an hour.