WHMCS billing based on the amount of emails sent

I need some help to think. I got WHMCS API and Mailgun API.

My server uses Mailgun to send out emails. I want to bill my customers based on the amount of emails they've sent.

With Mailgun's API I can get the amount of emails my customers sent, but how would I make WHMCS get this information and bill them?

Any help will be appreciated.