According to GeoNames, it's said to be 94390
Full line from FR.zip:
FR 94390 Paray-Vieille-Poste Île-de-France A8 Essonne 91 Arrondissement de Palaiseau 913 48.714 2.3628 5
But according to http://fr.wikipedia.org/wiki/Paray-Vieille-Poste, postal code is 91550
See my other post, there's an OpenData for postal codes for France. It would ease to verify geonames data
I've seen http://geonames.org/v3/ but I don't find the way to contribute to postal codes indeed. I could contribute if there's a way to do so !