The engagement letter as the basis of the invoice
In France, article 151 of the decree of 30 March 2012 requires a written contract with each client defining the assignment and each party's rights and obligations; it can take the form of an engagement letter. Your invoices follow its terms: assignment names, flat fee, extras.
Source : Légifrance, décret n° 2012-432 du 30 mars 2012, art. 151