Maybe Magento is not that popular in Japan. I suggest that you create account at Japan Post, require them the API document and hire a developer to do it for you. At http://maconeto.com , you can request a shipping module for only 25$
If you still searching for a Fully Operative Magento extension for Japan Post you can find it at our website.
Our Extension will allow you to use every shipping methods from Japan Post, International and local for every country on the world.
The extension feature our exclusive full consolidation engine for the order.
This extension feature also multi-locale support, and full customization of each shipping methods.
If you, like us, are in Japan and selling your products all over the world you may be also very interested on our Paypal Currency Enabler (PCE) which will securely enable every PayPal currency on your shop for every PayPal payment methods, without loosing or overriding the Magento Core PayPal module validation function and without having you manually setting every currency. Everything is automatic and “work out of the box”.
Naturally all our extension are IonCube or other PHP encoder FREE.
So you can freely adjust and modify the extension if have the need, or you simply contact us for apply the modifications.
Please fell free to contact us for any information and support.