I recommend the La Poste Lettre Recomandée option as it provides tracking and proof of postage/reception.
Lettre Verte Suivie postage with La Poste is the other option offered.

La Poste Lettre Recomandée (R1):

France: €7,40
International: €9,45

Lettre Verte Suivie:
France: €3,28
International: €7,30

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