Depending on your state laws, anyone doing work to a vehicle SHOULD be required for written authorization PRIOR to doing any work.
If they try to hold your truck or put a lien on it, you'll probably have to sue them for the excess repairs that you didn't authorize as well as any documented rental costs due to being without your truck while said repairs were done without your authorization. Don't let 'em hold your truck, they already sound shady & they'll probably cause more damage.
If they give you crap about it, just calmly call the cops & give them the story.
They'll probably still send you invoices for the extra work, but wait until you can talk to a local attorney for advice before you give in.
I watch a lot of People's Court.