Got a Text

I got a text today....it was from my ex.   He was thanking me for buying our son a ticket to TX.  The grown son.  I don't know how to answer texts like that.  Because I didn't do it for him.  I did it for my son.  For my father in law.  I did it out of respect for myself and knowing that doing everything that I can for my kids without guilt is a gift and something that I want to share with them.
But I can't text back, "you're welcome".  I want to say, "I did what you should have thought of doing."  I made the offer, I followed through.  I gave though I have less money.  Though I pay for their phones, car insurance, and clothes.  Though he gives some money, he doesn't have the idea that he needs not to spend...but to SEE them.
It frustrates me in a heartbreaking way.  Makes me see how he is.  I see how he sees him first.  He offers the rest if it's convenient or gets him something.  It was just....painful to see.  I'm glad my son gets to go.  Glad that I did it.  I'm just not glad to have my ex behaving as if I did it for him.  I did it for kindness.  For joy.  For peace.  Not at all for him.  More like....in spite of him.
Go figure that he just doesn't get it.  I can't say "you're welcome".  So...what do I say?  I just didn't respond.
blessings.