Just go to your bank, exchange the invoice total 
in Yen, and then put the Yen banknotes into an 
envelope (it's also recommendable to put the 
money into dark colored paper first) and mail it 
to the addressee via registered air mail.
No bank transfer needed, no post office needed, 
no credit card needed.
It's a very simple way to pay the seller (I often 
do it so as I have had similiar problems in the 
past).
Normally all sellers accepted cash payment if you 
ask them.

Hope this helps.