Cancelation is unfortunately badly handled by the companies very often. That shouldn’t be the case but you know French and the administrative stuff? One thing is for sure, the sooner you send the letter the better your chances to do it properly. That’s one of the reason why I like to do that process myself, I’m usually waiting for the confirmation from the company before making the switch to the new one… just in case! Another important thing: it is mandatory that an insurer (or a mutuelle) answer you within 10 days, otherwise they are not respecting the ‘ACPR recommandation’. ACPR being the equivalent of the FCA in the UK.