private car services

I have used private car services for pickup at CDG and been happy with them but that was always for trips when someone else was paying for the airport transfer. When I am paying, I take a taxi (or mass transit, but usually a taxi) and unless you need a car seat or an accessible taxi, there is no reason to pre-book one. All of that being said, I like Euro Jet Set but it costs €140 for an airport pickup for two (http://jetsetprestigium.com/location_voitures_limousines_autocar/transportation_executive_sedan.html versus a taxi which will cost either 50€ or 55€. If you want to try to pre-book a taxi and I am not sure you can do that at CDG for a regular taxi but if you can this would the place: https://www.g7.fr/en/book-taxi

Just follow the overhead signs to the taxi stand.
50 or 55 EU, depending on which address you're going to.
Hand the driver your address, with the Postal Code, so there won't be any confusion.
No fees for luggage in the trunk.

When you are ready to book a taxi back to CDG, here's a reputable company - book in English on website or call them. There's a small reservation fee for the convienience:
https://www.g7.fr/en/

Uber isn't allowed to pick up passengers in the street, or unless you have already paid in advance to go to the airport.
Uber isn't allowed to use the dedicated taxi lanes from CDG, so it will take longer and cost you more.

Thank you so much for both of these replies. Very helpful.

We used a company called Interservice Prestige the last time we went to Paris. I had prebooked the first trip and since I was highly satisfied, I booked the return as well.

Ile St Louis is considered right bank, so it would be 50 euros. Here's information on taxis to/from CDG.

http://www.parisaeroport.fr/en/passengers/access/paris-charles-de-gaulle/taxi/paris-cdg-taxi

I think it's a good idea to have the 50 euros written down if you don't speak French so that you can show the taxi driver you KNOW ABOUT the fixed fare so they don't get any funny ideas. Sadly we've heard of this a couple of times on the Forum. Either write 50 euros or print the information from that page.

Keep in mind for the return (from Paris back out to the airport), you usually will want to reserve your taxi before departure, which unfortunately entails another fee. 4 euros if you call for an immediate taxi, 7 if you call and reserve for example the night before.