Implement Custom Payment Gateway ( RTCart ) in PHP

I am building a site for my client who wants a payment gateway setup. I have already set it up with PayPal and I know the basic idea with it ( verify using the notify_url input field ) . But my 开发者_运维问答client wants his own merchant solution with it, RTCart, he already has an account and all. The problem is I can't figure out how to implement this on the site and verify if the process has been completed.

This is the Gateway menu:

http://farm6.static.flickr.com/5100/5535625463_aedc5d5dd5_b.jpg

Has anybody worked with this cart before? Or know how to verify if the payment is complete ?

I think your best bet with this one would be going with the support section of RTCart's website. It has code explanations and a setup guide, but their site does look a bit dated (to say the least).

Also, here's their contact info with support ticket system.