I don't know about packing peanuts, but they would probably be too loose in the box and not offer alot of cushion.....but I have used shredded paper or polyester pillow stuffing. The idea is to get the snake as secure as possible to prevent it from being bounced around inside the box during transit.

I label my boxes with "Live Harmless Reptiles" on all sides and the top and also have a common name/scientific name label that is printed a little smaller and stuck to the top of the box.

IMHO....Using a 'used' box is not a good idea for a test box. As part of the Fedex contract, you must have each different type/size of box that you plan on using to ship be tested and approved. If you send snakes in boxes that are not identical to those that have been tested and approved, you Fedex shipping agreement will be violated. Also, your box will be tested with weight loading just like it was new and photos will be taken of the used box by Fedex for you 'file'. So why not just spend a few bucks and get a quality REPTILE shipping box certified?