there are two (that i know of) companies processing VAT refund in France / Italy. Global Blue and Premier Tax as what Lufu said.
Some retailers will use one, some will use the other. If you go to the Global Blue counter at the airport, they will only process the ones which are on the Global Blue forms. The Premier Tax ones you will then need to post back. I didn't see a Premier Tax booth at CDG airport. One thing is try not to take the refund back in any other currency because if you look at the exchange rates they use at the booth (which doubles up as a money changer), you will be shocked at how bad the rates are.
In any event, what you need most is the customs stamp. If you get the stamp, you can also mail the thing back and get the refund in your credit card. It will take a while though and you take the risk that they will lose your form.
Remember to go to the airport early to process your tax refund. You don't want a situation where you're late and you don't have time to get the customs stamp because then you will not be able to claim your tax refund. And there can be a long queue there if you happen to come after some big group of tourist (e.g. the chinese tourist that you see sweeping the stuff from the shops like it's free).
Alternatively, you can also go to the in-town tax refund places (e.g. Galeries Lafayette) where you can get a cash refund there. However, you will need to leave your credit card details and ensure that you get the customs stamp and mail the form out, otherwise they will bill your credit card for the refund they gave in advance. The drawback to this is that the processing fee is slightly higher.
