Basically, it's just a little something extra that is unexpected. In letterboxing terms, lagniappe is often found in the form of a "quisp".
I've gone Postal! And, no, that's not a bad thing. I am trying my hand at a Newbie Ring Postal put together by Littlemonkey , which is where a group of boxers will share stamps and logs in the mail. When I peeled off the label on the first box that addressed the envelope to me, I found a little piece of a label with a stamp on it. It was a little surprise and I thought it was so cool, that I sat down and carved a stamp so that I can make my own quisps when sending any type of mailing to my boxing friends. It was nice that it was on a label with a peel-off backing. All I did was trim it a little and stick it right in my journal.
After doing a little searching through the Help files on Atlas Quest and reading some of the forums and asking questions from our Postal host, quisps can be a variety of small, flat, hand-stamped images. They can be on pieces of paper or cardstock. Some are even done on Shrinky Dinks. (Didn't you just love those as a kid?) Since they are kind of hidden, I wonder how many go unfound? I see that most Quisps are listed as undersized LTCs.
LTC??? Seriously? Did you think that letterboxing was JUST taking directions, finding a box, stamping the image and going on about your business? Oh no, no! There are so many little extras to this hobby and I keep finding new ones and I'm having a blast. A LTC is a Letterbox Trading Card. This is another item that boxers will trade back and forth. These are stamped images (often embellished) that are the size of a business card. You can send one (as lagniappe) in a Postal, you can get into LTC rings and trade with other members of the ring or you can bring them along to events to pass out. I'm sure there are other things to do with them as well, but I'm still learning.
In addition to finding the Quisp, this first Postal box had an awesome logbook made by doglvrs. It looked so complicated and cool at the same time, but I studied it and figured it out. If you come to Flea Fest in Snook on May 4, you will see my attempt at it. I think it turned out rather well.