Hello. Your plugin works very very well and is exactly what we're looking for. I purchased a premium feature and this is all great. We as programmers are trying to add Atos Sips http://www.sips-atos.com/fr/1/Accueil.html as a payment gateway for the plugin. This is the number one ecommerce payment solution in France. We've found some workarounds to make it work. There is also a plugin WPCB which has the intergration. http://wordpress.org/extend/plugins/wpcb/.
My question is where in your plugin files and database can we add to the payment gateways? Do you have any API documentation or instructions for developers? We'll of course send you what we do so you can perhaps just make it official. But any help or instructions would be helpful.