Show them your new car purchase record, if old car not on toll r

at that time, you can try explain this to them, and I think they could be nice enough to waive your fine.

I am not anywhere close to Houston or Texas, but I did a web search, one seems to need to register the new license plate to his/her EZ Tag account within 30days. During these 30days, I believe you can just switch the tag to your new car.

Good luck!