French letters? Perhaps, do you mean french ticklers?
No, French letter is an old slang term for condom. Here's a musing from Random House on why:
http://www.randomhouse.com/wotd/index.pperl?date=19980114
"letter" possibly refers to the type of envelope enclosure in which they used to be packaged, 100+ years ago.
OK.....
Found this...
In Britain a condom is also known as a French letter, much like the colloquial German word for a condom, "Pariser".
English 17th century tourists, travelling through France on their pilgrimage to the center of ancient culture that was Rome, came accross the town of Condom on the East coast of France. It is said that there they made contact with ingenious French shepherds who were making profilactics from sheep gut.
A trade then ensued, whereby the English gentry would eagerly await their letters from France - French Letters - with a fresh supply of condoms. The French aristocracy then learned of these useful items from their English friends and called them "Capote Anglaise" - English Raincoats.
Thus the condom came full circle, being made in France, being used in London, latterly in Paris, and finally being adopted the Germans as a Pariser.