When you share an invoice, Event Temple displays a default system email. You cannot permanently change this default text, but you can edit the email text each time you send an invoice.
For consistent messaging, you can create an Email Template and select it when sharing invoices.
To manage invoice content and behaviour:
Go to Settings → Invoice Settings under your Organization
Customize general settings, terms, numbering, labels, and online payments
Changes to Invoice Terms, Field Labels, and Footer Notes/Terms apply only to new invoices