I know there is a really good article on this site on shipping fish, which is a good place to start. It lays out the basics and has a lot of pictures.
Basically you will need a large box, something to insulate it with (I use styrofoam), newspaper or other padding, a fish bag large enough to put the fish in, another large fish bag so you can double bag him, and rubber bands to seal the bag. You can order fish bags online pretty cheap, or see if you can get some from your LFS. Mine is happy to spare a few bags when I need to ship something. You will want a bag large enough to have the fish covered in water and about twice as much air in the bag as there is water. If you have access to an oxygen canister, filling the bag with oxygen is better than just air. If not, an air pump for running a bubbler works well for filling the bag with air. As far as boxes go, the flat rate boxes are always a good idea - the fish and water can get heavy when shipping a larger fish like a piranha. I would reccomend only shipping express mail or overnight so that he doesn't spend too long in the box. Depending on the weather where you are and where the recipient is, I would consider a cold pack of some sort. Also, please be sure of the legality of shipping a piranha to whatever state your buyer lives in. Don't want to risk any nasty fines just for shipping the wrong fish to the wrong state.