What is a callback page, and how does it relate to billing?
Callbacks are used to verify that payment has been sent to a payment gateway. So a callback page is where you get sent to once payment is completed. These callback pages come with the Helm installation, so to use them you just need to set the callback link to be:
The callback page will vary depending what gateways you use.
So for instance, the WorldPay callback page will be:
However, you do not need to specify this unless your payment gateway requires it, since Helm tells the supported gateway where to post back to. If you use a custom link however, there are no callback options available as no variables are passed.