I always have to laugh at these arguments.
Isabel, youo will be fine. The IDP fulfills the French requirement for a translation.
As to having to have one.....
1) Avis makes their own rules. If they insist you have a permit, you need a permit.
2) Avis phone reps may not know the rules. They may be confused with license and permit. You can't drive with just the IDP. You need the license as well. What confuses people is understanding what an IDP is. It is simply a translation of your driver's license.
3) Regardless of those insisting they don't need one because their license translates into all languages including Martian isn't going to help you any when pulled over by a French police officer whose only English is one or two words. Do you really think they are going to know which states have the proper "format"" license and which don't?
4) If you get a ticket because you don't want to get an IDP--not pertinent in your case--what are you going to do? Fly back to France to argue it in court?
I never rent cars in Europe. Yet I carry an IDP in case I want to at the last minute. To me, it's a $20 insurance policy against a possible larger fine.
Everyone is free to do what they want.
Isobel.....you're fine. The IDP is the only thing you will need along with your drivers license. Have a great time in France.