I pay my ex the ordered child support. We live a good distance apart. She has primary custodial responsibility, takes the children to the dentist/doctor, and we usually split the out of pocket medical/dental bills 50/50.

We've had disagreements on a few of these because she never (not once) forms an agreement on which orthodontis/dentist/doctor and the treatment plan. She just signs up for treatment and then sends me the bill. I've asked that we be a little more frugal and shop around on the orthodontist (for example, one treatment plan ended up costing close to $7000, when at another place it could have been closer to $4500).

Now I am getting the bills from the dr/dentist sent directly to me saying I am the responsible party. I thought the only way they can do that is if I signed and if they have my personal information (SSN, etc). I never agreed to it.

I think my ex is just signing my name as the responsible party.

Again, I have been paying 50% and now she seems to be signing me up for all of it.

Can she legally just assign me responsibility if I am not there?

I was just at the Dr.'s office with my child yesterday. The insurance is under her dad's name, but I still had to sign as the person responsible for making sure it gets paid. So ya, I'm not sure how your ex is getting away with that exactly. I would call the office directly and ask them or ask to see the paperwork.

I'm quite sure the laws about this sort of thing differ from state to state and may be entirely irrelevant. I suggest you contact your attorney or some other legal service to determine, based on what's in your child custody agreement, if what is happening is illegal. You would, of course, have to document irrefutably that your suspicions are true and your ex really is doing what you think she's doing.

While it may be perfectly legal to stick your ex with your kid's orthodontics bill, it might violate your child custody agreement. This is not a question any of us here can answer. Go read the divorce/custody papers and then call your lawyer.

Go see a lawyer and ask that the child support sgreement be altered to reflect the additional expenses you are being asked to pay. If the judge agrees, she will take a hit on child support and that may convince her to at least work with you, although I doubt it. usually in these situations, the other side sees things much differently than you do.