It could well be added to every order the salesmen do – you are not charged for it, Motability won’t pay it (they only pay for metallic paint), but, as its “on the paperwork” the salesman gets his extra % commission. The scam/rip off is not on you, it is on the dealership by the sales staff.
Or its dealership policy on all cars, charged to the customer or not, since they neither mentioned it to you nor charged you for it. Maybe they add it and try to bill it to retail customers, you’d be surprised how many folk would grumble and pay up. It never ceases to amaze me how badly some dealerships treat customers and still expect them to return for another sale/service later.
If, on the other hand, they added it and then tried to charge you for it or insisted it was a part of the order then stand your ground, refuse to sign till it’s taken off the invoice, notify Motability etc etc.
In life, it's not who you know that's important, it's how your wife found out.