If you need to send an important letter in
I have just helped an English client to reapply for a British passport and to ensure paperwork will not get lost in the post I am going to send it by registered post (en recommandé).
Visit your local post office and ask for “envoi en recommandé”. They will give you a slip (see photo) where you will have to write your name and address and the same information for the recipient.
The post office staff will date-stamp the multi-part docket, give you one which will be your receipt. Once the recipient has received your letter, you will get another copy of the docket which will also be date-stamped. This will be the proof that the letter has been received. It will cost you around €5.