We also flew to Spain, hired a car, and then drove to France for our holiday. At the airport, we were asked for our address in Spain and we said we'd be staying in France. We were charged an additional €7.50 per day additional insurance. So beware. Maybe if you're spending a night in Spain at all and have an address, you can give that to the rental agency. In Northern Spain in particular, they always try to charge additional stuff for travel to France.