Custom payment gateway – GMBill (help needed)

Hello!
I’m working for a company that quickly needs several web sites up and running so we’ve turned to worpress-based sites. We’ve found only s2Member for user-management to fully fit our needs. …well, almost fully because we need to have payments done via GMBill.
I was digging in the documentation and reviewing code, to find my own way of integrating this, but I can’t figure it out. My idea was to make this integration as a plugin that then hooks onto s2Memeber for handling payments, but I don’t even know where to start. I’ve developed several web pages in PHP, on CodeIgniter framework, but I’m new to wordpress and plugin development (made only 2 plugins so far).
The integration needs to be really basic, no need for advanced user options like there are for PayPal. Is there anyone who may be able to help me with this, at least to get me started, or even create a small integration for me to develop further? And if so, at what price?

Do you know how GMbill communicates with your website? IPN?

I think you could start by looking at some of the other integration option code to see what they do. If I remember correctly, it seems like authorize.net function processed the payment then sent everything through the final paypal functions. I’d have to look at it again, it’s been a long time.

That might be a place to start.

Dave

It functions using credits, a virtual currency that users buy via GMBill.
Credit balance and purchases of premium content with those credits should be done purely on our side. But when user goes to buy credits, an XML order should be generated on our side, encoded and sent to GMBill with order hash through a post method. The rest is then up to GMBill’s system as customer is guided through their sign-up process. When customer signs in, the data which is returned gets inserted into our database with GMBill client application which they have provided.