I would give Avis' BA helpdesk a call - 0844 581 0124. I needed to check the details of a hire I booked in FCO this past June and I found their service outstanding. The guy even called the manager at FCO to make sure everything was in order.

On the additional driver question - the desk agent did initially say that there was a charge for the extra driver but their system only showed the discount on the final screen. So don't bother arguing about it until they get to that point.