Fiscal representation is required by all foreign residents not in the EEA selling a property over €150,000. The job of the “représentant fiscal” is to ensure you pay the correct amount of capital gains tax. The Notaire already does this, and even the Notaire will admit this new law is quite pointless, but it is now a requirement. It did apply to all non-residents until this was declared illegal under EU law and was stopped 1st January 2015.
The fee is usually 1% of the selling price. If you do not have a representative then the Notaire can choose one for you, though as this could be unnecessarily expensive it is worth choosing one for your Notaire.