I think the rep must have been mistaken. I highly doubt there's an aisle there when the website specifically says counter service.
When you asked about the "guaranteed vehicle," I guess he was confused also. That simply means they will have a vehicle for you, i.e. you don't run the risk of them overselling. You can't guarantee a particular car. Of course if GJT is run by a franchisee, all bets are off. Why don't you just book online?
-J.