What are billets in The Three Musketeers?

A billet is a short letter or a note.

In Alexandre Dumas's The Three Musketeers, Planchet brings two letters to d'Artagnan:

The one was a little billet, genteelly folded, with a pretty seal in green wax on which was impressed a dove bearing a green branch. The other was a large square epistle, resplendent with the terrible arms [seal] of his Eminence the cardinal duke.