I have exactly what you are looking for. I had a 15gb 3g that has a dead HD and will probably never work again. I had the box in storage, just pulled it out and it is in mint condition, looks like it hasn't left the store. It even still has the clear plastic "don't steal music" seal that comes surrounding the iPod in it. I will include the manual, CD, packaging, etc... That should be everything you need.
Would you be willing to pay for shipping? It shouldn't cost too much being an empty box so it is fairly light, but I am sure they will charge a small amount for packaging it up.