Add support for multi payment-method merchant gateways
I'm working on a payment gateway for Mollie, which is an aggregator of multiple payment methods (similar to 2Checkout). It supports one-time billing, subscription billing and on-demand billing using authorizations. The problem I'm having is that WHMCS doesn't seem to support this kind of module.
Since Mollie only allows me to modify customer details (saved payment methods, subscriptions, authorizations) using their API, I need to have a form in WHMCS that shows the customer their currently entered mandates (a mandate in Mollie is basically an authorization for me to use a specific payment method/account for subscriptions or on-demand charging) and modify them. This page would have a similar function as the credit card details page, in that the customer can update their details here.
However, instead of simply overwriting whatever data the customer had for their payment details with new info, as is the case with credit card gateways, I want to show them a different form.
The ideal implementation would be allowing me to create another function in my gateway (something like Mollie_update or whatever) that returns HTML code to be used on the page. This should be accessible without the customer having to go to an invoice.
TL;DR: Add a new page to WHMCS that allows payment gateways to display HTML content without having to modify existing templates. Should be accessible from the client area without first viewing an invoice.