If you want to send a parcel via EMS, SAL, Surface or just Airmail then its very easy, just go to the website look up information, go to post office fill out a form, stick to box, give to post worker, pay and everything is done. Sending a parcel domesitcally in Japan does seem a lot harder, for a start they don't have information in English on the website (just a link to the Japanese page) and as there seem to be a number of services I am confused to which one I should use, I tried to ask in broken Japanese but got nowhere and had to leave the post office with a box. Anyone got any tips on how to send parcels easily? (Knowing Expressed and Insured would be nice too).
For sending parcels within Japan you're better off using Kuro Neko (Mail Bin) which you can do at any Seven Eleven as well as 100's of other places. If you must use the Post Office though, then send stuff via YouPack. The forms have English on them just like the EMS / SAL ones so you should be fine there. Fill in the form, hand it over to the staff with your parcel then pay loads of cash. I send GT Pro with wheel on the Wii to a guy in Yamanashi today from Yamaguchi. It cost me 800 odd yen. Not cheap but at least it's insured. Yakumo
Mail Bin has size limitations though, which "Futsuu Teikeigai" (cheap service by the post office) does not. Yuupack is the more expensive / insured form of local parcel post.