Hello,
I am an expert in WHMCS with 7years of development experience.
I have created addon modules similar what you are looking for. Pushing invoices from WHMCS -> External billing via API. [url: https://www.freelancer.com/projects/php/whmcs-external-billing-api-connection/]
I have created 2 modules similar to this.
I can do this for you.
I follow best practices in coding, following industry standards. I follow WHMCS DBAL (Capsule), rather than mysql based queries, which is both WHMCS & php upgrade compatible.
Regards