La Poste, via sound Consumer Service, provides you with an online complaint form to explain the problem you are experiencing in the delivery of your mail.
DGCCRF is reachable online, by mail or email, by telephone (at 3939 with an interactive voice server from Monday to Friday from 8.45am to 5.30pm - €0.15 the minute + call price). It will inform you about your rights and about the different remedies available.
The Arcep allows you toalert about malfunctions that you encounter in your relations with the operators (fixed, mobile, internet and postal). This will be used to quantify problems and to better target actions to professionals. It also provides appropriate advice and guidance on remedies.
Be careful, the Arcep platform does not offer a personalized follow-up of your case. The remedies available to you will be indicated at the end of the alert.