From the MarketPress Page:
MarketPress was built from the ground up to be an infinitely expandable framework. There are simple fully-documented plugin APIs for adding your own custom Shipping or Payment Gateway modules. Don't get locked in to the current features of another WordPress e-commerce plugin! It's a cinch for any developer to create their own Payment Gateway or Shipping plugin by overriding our documented classes. Just drop it in the appropriate plugin folders and they'll show up in the admin interface.
Where can I find this API documentation?