lagniappe n., ((lan-YAP, LAN-yap) [From Louisiana French, from American Spanish la ñapa (the gift), from Quechua yapa (something added).]
a small gift given a customer by a merchant at the time of purchase (e.g., a 13th doughnut added in with a purchase of a dozen);
broadly: an unexpected benefit, something given or obtained gratuitously or by way of good measure.
"Of little consequence, a tiny lagniappe and a green salad came compliments of the house."
-- M.H. Reed; In Ossining, a Restaurant With a Past; The New York Times; Sep 23, 2001.