You can send an email when viewing a RECEIPT or INVOICE transaction.
When payment is made a RECEIPT email is sent. Template is RECEIPT_TEMPLATE_HTML in Site Config, and CLIENT_RECEIPT_SUBJECT (subject line)
You can resend this email when viewing a RECEIPT transaction.
If you click on “email” when viewing an INVOICE transaction, one of two templates is used:
INVOICE_TXN_TEMPLATE_HTML (for members)
INVOICE_TXN_TEMPLATE_FOR_PUBLIC_HTML (for public).
Exception is Subscription and Voucher invoices which use their own template.
When sending an invoice related to a booking, consider instead clicking through to the booking screen and using the email button there, which sends a more informative email about the booking. The email template there is determined by the booking status.