Since you have the Japanese address, why don't you 
just print it out and stick it on the envelope?  
As long as you write the word, Japan, belows the 
Japanese address, then the post office here 
will know which country to send.  And the 
post office in Japan will know where to send it 
by reading the Japanese address.  I have done it a 
couple times like this before, it worked 
out smoothly.