I am sending small items, less than 50 grams, from France to the US mostly, and normally use "lettre Prioritaire" which takes 1-4 weeks for 1.78 Euro. I just sent something "post export" for 6.73 Euro because they said 4 days to California. (without tracking) Has anyone else used "post export"? Are the expected delivery times accurate? Is this similar to the US Priority mail? Is it worth the extra 5 euro? Thanks!
After a search on the french Poste site, you can see on this page (in french) that Postexport documents is an offer for a pre-stamped pack/enveloppe. The price therefore includes this package but the delivery system is described as Lettre Prioritaire, so not different from your 1.78 € stamp for a letter under 50g.
On the same page, you can see that there's an offer called Export Suivi, that you can add over a letter already stamped as a Lettre Prioritaire to benefit tracking of your letter, with no mention of a faster delivery.
So my feeling is that you will not benefit of a faster delivery by using Postexport, neither Export Suivi.
This document (in french) mentions a 4 to 11 days for delivery of Prioritaire in the USA outside LA and NYC.