The Etymonline entry for invoice states (source):
apparently from M.Fr. envois, pl. of envoi "dispatch (of goods),"
Although my French is pretty poor, my understanding is that the 's' is silent. We don't pronounce the 's' in hors d'oeuvre (usually), so how did the 's' in envois become vocalized?
Two ideas that come to mind are that 1) people read the spelling without knowing the proper pronunciation and it caught on, or 2) a "rule-maker" decided the 's' shouldn't be silent (and turned it into a 'c'). If the latter is the case, are there some other examples of this?