How do I configure who receives invoice e-mails?
For personal accounts, invoices can only be sent to the main e-mail address listed for the account. You can change the e-mail from your Account Settings, but this change would take effect on the entire account, not just for billing e-mails.
For Heroku Team accounts, anyone with the role of Admin will receive billing-related e-mails. You can check who the Admins are by navigating to the Team view on Heroku Dashboard and going to the Access tab.