When customers paid enough to cover FedEx, we sent the package that way. We always will ship FedEx if the customer requests (and pays) for the upgrade which is usually only a few Euros more as we have special rates.
If we absorbed the cost of FedEx, when a customer has paid enough for just mail, it would erode our margins- which are already very low on this product.
Your trackable package was posted on the 29th December (6 working days ago)- the package will require a signature and you may have to pick it up from your local post office, so keep in contact with them. Often in France, in our experience, the local post office does not tell the recipient that they have a package waiting for them.