WHMCS payment gateway: recurring but not credit card

I’m working on a custom WHMCS payment gateway module for the Mollie payment gateway. So far I integrated one-off payments with no issue, but I’m having some trouble implementing recurring payments.

The flow for either a subscription or authorization to charge the customer on-demand differs slightly.

The flow for creating a subscription is:

1. Create a customer profile in Mollie
2. Create a mandate by performing an authorization payment – used to link a payment method to a customer (must be a payment of at least €0.01, requires user input)
3. Add a subscription to a mandate, telling Mollie to charge the associated card x amount every x days/weeks/months

The flow for authorizing the merchant to charge on-demand is:

1. Create a customer profile in Mollie
2. Create a mandate by performing an authorization payment – used to link a payment method to a customer (must be a payment of at least €0.01, requires user input)
3. Merchant sends a payment request to Mollie for every new invoice

The problem that I have is that Mollie supports multiple payment methods per customer. While WHMCS has built their gateway module system around single-method gateways such as Stripe.

What I want to do is replace the credit card management page with a custom page (preferably generated in the module, without modifying any templates) that allows customers to manage their Mollie details. This would include seeing active mandates and adding new ones (again, a mandate is basically an authorization for me to use a specific payment method for subscriptions or on-demand charging).

I could not find any way of doing this in the WHMCS documentation. Is this at all possible, or should I accept that the only way for customers to see the pages generated by my module is for them to go through an invoice first.

Answer

You are going to need to create two modules that work together, a payment gateway module and an addon module. The addon module will allow you to create client side pages where customers can enter agreements / adjust agreements and then use the _link function to display a form where clients can pick which agreement to use when paying the invoice.