Terraform Cloud generates and sends monthly invoices for all Terraform Cloud organizations that are subscribed to the Standard plan.
These invoices are sent out via HCP to the email address configured in the HCP organization's payment method settings.
If you need to change the e-mail provided in HCP you can do it from here:
Billing -> Payment methods -> Manage -> Edit credit card -> update the Billing email address at the bottom.
You can find a more detailed guide in the following article.