Here is what you need.
1. A 12 1/2in x 9 1/2in Flat Rate shipping envelope, price $0.00
2. A strip of plywood* ripped down to a width equal to the shortest dimension of your box, price $2.00 or free (scrap)
3. 8 wood screws 1 1/4in length or longer, price $1.79 or free (scrap)
Table Saw for ripping the plywood (not needed if using a 2x4)
Miter Saw for cutting the sides to length
Philips P2 bit (or T20 if using DeckMate, the only wood screws I use)
Counter sink bit (recess the screw heads, it's prettier)
, Wood Spade
, or Auger
* You can opt for a simple 2x4 if you don't have a means of ripping the plywood, but it won't be very useful for stuffing.