How can we improve WHMCS?

I recently had an issue where a direct debit went through WHMCS successfully as PAID and then got a notification from GoCardless right afterwards that it had actually failed as the direct debit was never setup, meaning I wouldn't get paid.

Turns out that the way GoCardless works is that there is no payment confirmed process from the bank.
All they do is wait to see if payment fails, if not, they consider it paid and send the callback request.
So if the bank takes longer than expected to send the failure notification, this results in you have a paid transaction in WHMCS, which you need to roll back manually, as WHMCS does not process such requests.

One of the reasons this happens is if the customers bank is not actually supported by GoCardless, and so the mandate never gets setup.

The following has been recommended by Gocardless

"It would be worth utilising our bank details lookups endpoint here to complete a modulus check and a reachability check."
https://developer.gocardless.com/api-reference/#bank-details-lookups-perform-a-bank-details-lookup

Presumably this would verify if the bank is supported prior to trying to setup the mandate.