and get this resolved and in writing. Then you don't have to worry. Go in with all the information. You shouldn't have to stress about all this. It should be cut and dry and mediation will help with that.
As for the "loan paperwork", I would tell her that she can pay her 1/2 how ever she wants but you will be paying yours monthly directly to the ortho. I have really found it is the best all around to just pay your portions to the provider instead of to the BM. That is what I always did in the past. Then you have proof that it was paid and you don't have to deal with BM about it all the time.
I however, wouldn't send the certified letter to the ortho. She has custody of the kids even though DH has joint legal. That really doesn't mean much. If she can prove it is medically necessary then the court isn't going to hold it up and wait for when you can pay.
As for the "loan paperwork", I would tell her that she can pay her 1/2 how ever she wants but you will be paying yours monthly directly to the ortho. I have really found it is the best all around to just pay your portions to the provider instead of to the BM. That is what I always did in the past. Then you have proof that it was paid and you don't have to deal with BM about it all the time.
I however, wouldn't send the certified letter to the ortho. She has custody of the kids even though DH has joint legal. That really doesn't mean much. If she can prove it is medically necessary then the court isn't going to hold it up and wait for when you can pay.