Make sure if you're buying fuel at automated service stations that your card will be accepted in the machine. We did a 2 car convoy down to Brittany a few years back armed with about 5 different cards between us, and not one would work in their machines. Eventually (with my mate having no fuel left to carry on) we managed to find an agreeable frenchie who took some cash and allowed Nige to fill up on his french bank card. And these were Visa, Mastercard etc. cards.
Things might've changed now, but it was a real sh*tty start to a badly planned holiday.