Its insurance - from the La Poste website:
International Declared Value meets your need to insure your consignments of valuable small goods. The declared International Value allows a secure delivery of your shipment, a delivery against signature to the recipient and compensation for the amount of the value of the content declared at the time of deposit, in case of loss, theft or damage.
International Declared Value allows you to secure your shipments as part of your management relationship with your customers and suppliers.