You can add your own payment gateway by the setting below.

        // Create a basic cart
        var cart = $('#cart').DCAJAXPaypalCart({
            width:600,
            height:400,
            paymentOptions:{
                server:"http://YOU_PAYMENT_GATEWAY_URL.php"
            }
        });

It will post a set of item and shipping by “POST” method to the gateway.

The var name is same as the item and shipping variables in paypal.

https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_Appx_websitestandard_htmlvariables