Hello from Wisconsin,
I always start my search with Auto Europe, and they almost always hook me up with Europcar. On occasion I have tried the links with the airline I have used, or other search engines, but I have never been a successful as with Auto Europe. Last year for our Czech trip I searched independent rental companies, still went with Auto Europe's suggestion. One thing about AE is that they are up front with 'hidden costs'. A hidden cost would be like a daily road tax. All cars have to pay it, but not all rental agencies mention it until you are at their desk.
Have you considered taking a train into a city i France that you want to see, spending the night, and then picking up your car and returning to the same location so the rental is all in one country? I didn't find the drive from Barcelona to Perpignan that great. Train searches can be done at dbahn.com.