About the Ex

Yeah...not seeing how that is 50/50 custody--except maybe financially--on paper. And clearly since you are the only O. to travel, you incur additional expenses. He's still holding power over you. That stinks.

After reading your other question's So What Happened, I've gotta say, the "nervousness" you fell around him is fear. It's a conditioned response that happens when you see him. He is a bully and an abuser.
He has talked you into giving up custody of your child so HE can save money on support. This would seriously make me wonder whether he cares for his children at all. Especially since he is so casual about if & when he sees his daughter.
You are really taking the high road here, with respect to bearing the burden of these visits financially. What self-respecting man or father would allow that? I'll tell you what kind, a controlling SOB, that's what kind.
I'm sorry you ever had this monster in your life. I pray his nastiness has not extended to your children and I pray you now have a husband that value's your worth as a human being! God bless.

Your kids have been old enough to decide where they want to live/visit for YEARS. They don't HAVE to go with their father.

:(

why not pay for a train trip for your son to come up on the weekends to the nearest town? Or a cab, or bus, he's old enough to navigate on his own? It would be cheaper and hed get more time with you,
Does she want to see her dad? if shes indiferent leave it up to him to see her, or send her on a train or bus if she wants to. Also who moved away? if you did maybe he feels that's why you are obligated to provide transportation, although you should meet half way and switch off kids

Are kids not allowed to get a license @ 16 in TX?

When I was 17, I went where I wanted, when I wanted [as far as visiting family] because I had a license and a car.

ETA- When your son gets his license, he can be the transporter for his sister! Give him some gas money and you don't have to see your ex at all!

AND if she doesn't want to see her dad all that often.... She doesn't have to. She's 16!!!

"Custody" is not the same as "parenting time". It should say specifically in your papers who has the child and when the other parent has time with the child. Normally, if its your turn to have your child then YOU come and get them. When time with that parent is over, then if the other parent wants the child back then THEY come and get them. Its called "receiving parents pick up". That way its "fair", you both are doing the driving. This is a simple fix: go to your courthouse and get the form to "modify parenting time". Then fill it out to say that the "recieving parent" is to pick up the child for their parenting time. Good luck.