Hey Philip,

We can't entirely emulate BeginPaymentForm on the server side as depending on the payment provider being used, some of them return javascript that needs to be executed so we can't just return a URL to navigate to.

If you know the payment gateway, and you know it returns a URL and you want to bypass everything Vendr does, you can call order.InitializeTransaction() to give the order an order number before redirecting to the known providers URL.

My generally recommended approach though if you want it to work for any payment gateway is to do what we do on the Vendr.net site and introduce an interim page after the single page checkout that displays a brief message "Redirecting to payment gateway" then on that page, render the BeginPaymentForm and use some javascript to automatically submit it. This is ultimately the JS that we use for auto submitting the form.

setTimeout(function() {
    var form = document.getElementById("paymentForm");
    if (form.hasAttribute("onsubmit")) {
        form.dispatchEvent(new Event('submit', { cancelable: true }));
    } else {
        form.submit();
    }
}, 100)00)

Hope this helps

Matt