How did "invoice" end up with an 's'-sound?

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?

Solution 1:

The OED takes a whack at explaining this as follows:

Etymology: app. orig. = invoyes, pl. of invoy, corresp. to 16th c. Fr. envoy (now envoi), f. envoyer to send: cf. Fr. lettre d’envoi letter of consignment, invoice.

Inferentially, this derivation is satisfactory, both as to meaning and form. In‑  from Fr. and earlier Eng. en‑  is usual; and the writing of  ‑ce for the plural  ‑s is found in other words, as dice, mice, pence, in some of which also, as accidence, bodice, dace, truce, the resulting form is treated as a singular. But the historical record is not complete: the examples of invoy, invoyes, are scanty and not very early, and an earlier envoy in this sense is not exemplified.

So it seems that we do in fact get  ‑ce words from things that were originally  ‑s words. However, in this case it sounds like they aren’t entirely convinced of the matter, either — which puts you in fine standing if I do say so myself. :)

Solution 2:

As the French spelling suggests: the s was originally pronounced. French spelling has a lot of 'ghosts' of former pronunciations in the same way that English does.