Hello Lawrence
the manuscript mention "par voye de Marseille", reminds the later cancel "VOYE DE MER PAR MARSEILLE" who is said buy G Dutau p417 of his book to have been associated to the sanitation service, could suggest it has been disinfected; however it seems to have been written by the sender here
being very early, (earliest disinfected date in Marseille is a 1601 letter from Alep) , France did have a document "Arrest de la cour du parlement ( de Provence )" dated 1629 , mentioning that coversshould be "perfumed" with fumes of laurel, geniper, and rosmarine OR immersed in vinegar ; I have a "reglement sanitaire" booklet of 1789, mentioning the same methods, in addition to slitting
I'd be glad to further discuss with you , please email to roland.goutay@sfr.fr