My friend's 2017 tailgate was stolen out of his driveway. $6000 insurance claim later it was replaced. I think his had a lock, but it wasn't tied into the electric locks of the truck (on an upper level trim package too!). I have a Tradesman Ram (bottom of the line) and the tailgate locks with the doors whenever you hit the lock button.
OP, I would 100% go after the repair shop for replacement. Your insurance will go up as a result, which isn't fair to you. They have an obligation to maintain your property when they are in possession of it.