David,
This email template is sent out in two places. First, from the Customers>Receive Payment screen. It won't be emailed to all contacts, only the contact that is specified on screen (in this case... me!). Also, the email will only be sent if Email payment notification when saving? is checked. See my screenshot below:
Secondly, a contact can also trigger the email to be sent to them if they make a payment through the Contacts Portal. You will need a Payment Gateway setup in order for your customers to make payments through the portal. If you receive a payment through Quickbooks, and not through Method, then the Method email will not be sent.
If you're interested in sending the email to multiple contacts, then you would need some customization done on the Received Payment screen and the Contact Portal.
Hopefully I addressed everything. Let me know if you have further questions!
Jason