No pre-payment required on mail sent to France at the time, so no stamps were required.
The letter mentions an enclosure that under the circumstances would double the postage (if over 1/2 oz). So, if going by mail from New York to Boston the letter is short five cents postage. However, since the missive was going out of country the letter wouldn't even be accepted because the PO had no opportunity to collect the postage due.
Most curious. The dateline is December 15th while the Britannia departed Boston on December 16th. So, under the circumstances if posted in New York the letter would never make the ship sailing the next day. Obviously the sender had no intention of mailing the letter and hand-carried it to Boston, or had it boot-legged by another individual.
The stamp is a first issue orange-brown. However, burning a five-cent stamp on a letter to be hand-carried seems suspicious. People do stupid things, particularly inattentive clerks. So, it's not impossible the stamp could have originated, but not very likely either.