I'd recommend looking at Degica's Komoju API documentation, which provides a full interface for creating payments on all major konbini chains. [Full disclosure: I work for Degica.]
The API is not public (yet) but if you are interested in using it or want more information you can contact Degica directly. The system is very flexible in terms of customizing the konbini payment receipts, etc., and the API is clean and straightforward to use and includes other local payment options as well (wallets, bank transfer, etc.)