Are you flying with RA?? We're just back from 9 days flew into Carcassonne and hired a car from Hertz through RA, it worked out the cheapest, and we have had no problems with them. FWIW, you need a credit card to book the car, and the drivers licence must be in the same name as the credit card (another bloke at the desk was having a problem).
Check out http://www.rentacar-europe.com/ They consistently come up with excellent quotes as a broker. I've used them at least half a dozen times in France and Italy and consistently have found them v good.
Spent some wonderful time in Carcassonne some years ago driving around the Languedoc region and beyond... you'll thoroughly enjoy it....