I don't know what the rates shipping from Japan to US are but i know shipping from US to japan it generally costs 90-120$s to ship a 15-20lb box to my cousin.
The added benefit of shipping stuff back though is tracking(i think Japan offers it anyways). If you have a duffel bag filled to the brim with imports and the airlines decide to lose it you are SOL
Also another tip. Leave all non-essentials behind. When you come back leave some or all of your clothes, toiletries, etc there. You can always grab another shirt or jeans at the store when you get home
Stuff your carry on with goodies from your trip.