Had a similar thing happen to me: got a notice for a car I'd sold a month before... and the notice had dozens of "incidents" over the course of a year... several from AFTER when I sold it, and I had a TOLLTAG the entire time I owned it....
Here's the thing:
When they do not see a tolltag (it happens sometimes even when there IS a tolltag), they snap a pic of your license plate... If your plate number is in the tolltag DB, they assess you the toll to your tag (and you never hear about it). If not, after some time, they search the TxDOT DBs for your owner info and send out the notice. MY deal was that the state issued me new tags about 18 months before that. I never thought to go update my tag info on the NTTA website.
So, I emailed them and explained I didnt mind paying tolls from when I owned it, and wish they'd have notified me sooner. They said to change the license plate in their website, they'd assess the tolls for the period I owned it, and I faxed them my bill-of-sale ... to let them pursue the new owner if they wished.
Anyway, get more tolltags than you need and keep all your vehicles' tag #s uptodate in the ntta website. Then, you can switch cards around all you want (I know cuz I do it). Where you'll get "busted" is if a tolltag doesnt "read" in a vehicle thats not in their tolltag DB.
Call 'em, or email 'em and explain... calmly... they'll probably work with you.
What *I* hate about NTTA is that on Dallas North Tollway in the evenings.... the tolltag lanes are slower than the Exact Change lanes. I just go thru them and rejoin the main lanes... leaving the dorks that panicked to cross the white line to get into the TollTag Only lanes a mile or more behind. The design of these roads is horrible.
Oh, and the Bush... all the money they spent on landscaping... I was skeptical... but now, I can honestly say "I like the Bush well groomed and kept".