During all of my recent trips i send several packages to my home address (Europe).
I go to the local postoffice (plenty around) to get a carton box. They sell boxes in several sizes (I also picked up a box from the street as there are in the in shopping area's in the evening always boxes laying around on the sidewalk to be picked up by the garbage collectors. However be careful not to 'steel' boxes from the homeless!)
At the postoffice i also take a couple of forms that you need to fill out (so i can fill them out in the hotel and mess up one or two). The form is in English. The postoffice employee can also make available a list with the prices for the several sending options).
A good start on the internet is here:
http://www.post.japanpost.jp/int/service/compare_en.htmlI always go for the lowest fare (surface mail) as it does not matter that it takes some time (in general less than 6 weeks) and i do not want the parcels to arrive at my home before i get home :-). Also in most cases i sent stuff home that i should not have brought in the first place!
Some info on the form to be filled out.
I always use the hotel address as my sending address. This is because there need to be an address. (They do not accept it without an address)
I never take insurance. Sometimes you need to take risks in your life (and i never sent expensive stuff). I did sent over time maybe 15 boxes and never a box was lost.
In case they can not deliver you need to give a forwarding address. I use the same address as the normal address (=my home address).
I never list all the individual items (no room for that on the form). I group them in 4-6 groups like: clothes or books/pamphlets, CD's etc and give them a very low value. I always mention on top of the list that it is surplus travelluggage. I am not sure that it works but so fare i never had to pay import duties in my home country.
As you do not have scales you can not give the weight of the individual items (in my case) groups so i just give an estimate.
When you drop the parcel in the postoffice you can ask for some extra tape or rope/string to secure the package more. They are always very kind to give this to you.
Sometimes they ask if there are no forbidden items in the box. And ones they asked if there were batteries in a electronic gadget that was in the box (apparently some batteries are not allowed).
They check all fiels of the form (if they contain data) and weight the box.
Note the opening times of the postoffices. I think (not sure) all offices handle boxes. In the bigger cities there are also postoffices that are always open. (I needed this ones when i discovered that just before leaving it was a national holiday and the regular offices were all closed).
Have a nice trip