Sometimes he gets backlogged and sometimes the 
seller gets backlogged and sometimes both ... I 
remember some items taking up to 3-4 weeks to 
arrive to Sato, so he couldn't issue an invoice 
until then.  There was also an incident when a 
seller disappeared for 2-3 months which resulted 
in buyers calling the police to investigate ... 
naturally there was nothing Sato could do then.

I just e-mailed him yesterday and he replied 
back promptly.  My guess is that your item 
simply hasn't arrived yet so he can't really 
respond to your e-mail until then.

I recommend just to continue e-mailing Sato 
politely once a week (his patience wears thin if 
you e-mail him frequently when he can't do 
anything about it).