A lagniappe is "a small gift given to a customer by a merchant at the time of a purchase", or more broadly, "something given or obtained gratuitously or by way of good measure." It can be used more generally as meaning any extra or unexpected benefit.
"We picked up one excellent word – a word worth travelling to New Orleans to get; a nice limber, expressive, handy word – 'Lagniappe.' They pronounce it lanny-yap ... When a child or a servant buys something in a shop – or even the mayor or governor, for aught I know – he finishes the operation by saying, – 'Give me something for lagniappe.' The shopman always responds; gives the child a bit of liquorice-root; (nb...)": Mark Twain, Life on the Mississippi (1883)
A dozen is a grouping of twelve.
A box of a dozen doughnuts
Half a dozen chargrilled oysters