you can't make a code for a single print, unless you copy that one image into a gallery and mark it differently (you'll want a new upload). otherwise, go to behind the scenes and there should be a coupon button for discount codes. set the amount of times it's for (1 is one time, but as many images as they want), and then the rest. explain to them it's only discounting your mark up, not FAA's.
---Mike Savad