In Mackerel, a notification email is sent out every month, at the time of the first completion of payment, to notify the usage fee settlement amount paid by credit card.
Initially, this email is only sent to the account of the organization owner, but optional email addresses can be added. This can be done from the "Organization Mail Addresses" section in the “Organization’s Settings tab”.
Input a mail address, enable "Plan updates", and press "Create". Making this change is only possible for users with administrative authority or higher.