From FRANCE on Your Own Newsletter (Sign up -- it's a great source of free information about France):
hidden credit card fees and money-saving tips that every traveler should
know about before boarding a plane for a foreign destination. If
you plan ahead, you will save yourself money, time and aggravation.
the most important phone call you may make before departure is to your
credit card issuer. Ask them what coverage you have if you use the
card to rent a car. If you will have full coverage, do not take
the rental agency insurance. It could cost you as much as the car rental
fee itself, and if you take the agency insurance, your credit card insurance
will be invalid.
We hope this information has been useful, and we suggest you do your own research on the credit cards you plan to take to France on your next trip. [Our apologies to readers outside the US for whom some of this information does not necessarily apply. We have no interest, financial or otherwise, in the credit cards suggested above nor with Rail Europe.]