You will be able to get all of your Paris receipts stamped and mailed off since you have a ticket leaving the EU to Switzerland.
I had a similar scenario. Finished a cruise out of Rome and did some shopping. I had a flight to Paris but with a short layover in Zurich so the customs agent just stamped all of my receipts even though I said I would do more shopping in Paris.
Not sure - good question. I suggest asking the sales person when making the purchase. And the sales person will fill out the form for you regardless. You just need to sign where they show you at the time of purchase.
One last point to emphasize - YOU MUST MAIL the validated forms before leaving the EU at the airport. If not, I have read it is a real hassle to do it from home.
You have to do your detaxe/VAT before you fly back to your home country including the customs stamp and drop form in mailbox.
If you're flying to Switzerland (probably ZRH which is a great airport for shopping by the way) from CDG, so a CDG/ZRH/MXP routing, you can detaxe all of your Paris purchases at CDG before you depart to ZRH. CDG has kiosks and it is very easy. If you also shop in Milan you would do your Milan purchases at MXP before going home.
If you take the train from Paris to Lucerne and then Milan then you'll do everything at MXP before you go back to your home country from MXP. Technically you can stop on the train somewhere and detaxe between France and Switzerland but it sounds like a pain.
If you do the reverse, MXP/ZRH/CDG, then you can detaxe any of the Milan purchases at MXP. What you buy in Paris you will detaxe at CDG before you return to your home country.
Thank you Everlong, pacificagal and HavLab! These information are really useful for my planning.
HavLab, you mentioned that I can do the detaxe everything at MXP.Thus, can I confirm that there is no need for me to do the detaxe in between France and Switzerland if I'm travelling by train from Paris to Lucerne? So I will get the customs stamps for all my purchases in these 3 places and drop the forms in MXP?