The last time - did you tell them?

If you don't tell them and you have an accident or theft of vehicle in another country, presumably your insurance may be invalid, except for the minimum level of cover required by law. Often the extra charge is to cover recovery when you break down, as a separate policy must be taken.

Wombatt, That time we used kemwel and we would have stated we were going to France, if we had been asked. I don't remember even being asked about it. This time the question was included in the quote and the crossing border card must be requested at the time of making a reservation. Reading the terms and conditions for Spain it states that if you take a car across a border and don't have authorization your car insurance is invalid. Deborah

Alec, I wondered if this is something recent between Spain and France borders. We have driven from France into surrounding countries on many trips and never had to pay any additional fees. Deborah

Depends on the rental company. Some give you freedom to drive into all neighbouring countries, viz France, Portugal, Gibraltar and Andorra (but not North Africa), while others make a charge for cross border card, i.e. permission to take your car out of Spain. Many companies don't allow taking expensive cars into Italy for theft risk, and there are further restrictions in Central and Eastern Europe's former Communist countries, at least for some of them.
I have come across restrictions imposed by Spanish rental companies, esp smaller local outfits. I suppose it's to do with the kind of insurance policy they have and level of risk they are prepared to accept.

Thanks for that information Alec. I did a comparison of renting a car out of Madrid vs Barcelona with Economy Car Rentals and there is no charge for a border crossing card with a Madrid rental but there is with a Barcelona rental. Deborah

That sounds like it's just because they don't think it likely at all for people renting in Madrid to be doing border crossing to France. Of course some could, but it would be really likely from Barcelona.