The practice where my child saw a therapist years ago screwed up the insurance reimbursement and then tried to make me pay for it. By then my child's therapist had left the practice so I didn't have an ongoing relationship with them. I just didn't pay the bill; I wrote them and said I wasn't going to pay it and they said they'd send it to collections. I thought given the amount, it would cost them more to do so and even if they did, I'd just ignore the collections and my assets are basically protected from any claims so I don't care.

If I were you, I would write back and say that you do not owe that amount and you won't pay anything until the bill reflects the proper amount. I did file a complaint against the therapist but at least in my state, they don't handle insurance "disputes." That therapist charged me for sessions that had been reimbursed by my insurance; I thought that was insurance fraud and they should care about it.