Challenge it. You did pay and display
You just didn't stick it to the window and the ticket blew off the dash.
After all, you have a valid ticket.
Personally, I'd just ignore them and see how far they can get with their threats, remember, it has to go to county court before any enforcement can be undertaken. To which you will get an invite and an opportunity to present your ticket.
I wouldn't pay them fook all.
That's providing it's a private firm. If it's the council then get your appeal in sharpish, because if you miss it and it goes to judgement. You will loose.