Thanks to everyone who responded to my 3+ days in Parisyou were most helpful. Now I have another question: We've always rented cars when traveling in France & Italy, but have never gone from France into Italy. This year we'll be driving into Italy after being on the French Riviera. Will we need any documentation besides our passports, US driver's licenses and IDP's? I'd really appreciate any information from any of you who've made such a crossing. We want to be prepared. Thanks so much!
I don't think you will even notice the border crossing except maybe suddenly everyone is speaking Italian.
If you pick up a car in one country and return it in another, there will be a very high charge for this. If you return it in the same country you picked it up, there is no extra charge for driving it from France into Italy.
Schengen Zone. It's like driving from one state to another. No checks at all (unless you're driving a tractor trailer not registered in the EU, but I'm assuming that won't apply to you...).
Make sure your rental company will allow the car to be taken into Italy, Not all rental companies do.
Driving in Italy: In the country on the freeways they are well behaved. In the cities driving is an imressionist art form. The English drive on the left. The French drive on the right. The Italians drive in the shade.