The letter is in French and appears to have been sent from New Orleans, LA.
Cover bears manuscript note: "By the first steamship for Liverpool" and is addressed to Nantes, France.
Red boxed cancel "SHORT PAID", indicating the postage was underpaid.
Black "NEW YORK/3/AUG 15" postmark.
Black "ET UNIS SERV./27 AOUT 60", and on verso: "NANTES/28 AOUT 60".
Old typed note with cover: "The 1857 U.S.-French Treaty rate was 15 cents. The sender used a 5c stamp in error presumably expecting the letter would go by British open mail as pre-treaty rate. The New 3 York indicates debit of 3c to France. French Exchange Office postmark applied in the traveling post office Calais to Paris."