Postal information:
Too much items in your suitcase::

If you have too many things and if you are asked to pay for the
extra charge for your overweight luggage, you have to run to the
post office right next to the departure gate, behind the
check in counter however not everybody has so much time to
spend for postal case.
Better sending your things from a local post office not at the airport
so that you still have time to pack the box.
Postage is much cheaper than the airline extra luggage fare.
Box:

Sending your items at the last days of your stay
please, avoid using boxes of agricultural products as it is forbidden
to accept for some countries like Australia for example.

Better buying a box at the post office to be sure, it is only
a couple of 100 yens, it is better to spend few yens than
wasting the postage of couple of thousand yen that was
refused to accept and returned to Japan.
To send you have to pay the postage again.