To set default email templates, open Preferences and click on the Email tab. In this window you are able to set the default email template used for a number of actions. In our case we are looking at the Default Invoice field

Clicking the grey box  to the right of the field will open the Email Templates viewer. In here you can see a list of the available templates as well as edit or create new ones. To create a new one, click New Template

In the new template screen, you first want to select the category that the template fits into. In this example, there is no category for Invoices, so we will create a new one

Add a description of what the template is for, the subject of your email and the body of your email. Merge fields are available from the top bar to bring through information such as todays date

Please Note: Merge fields are not applied in the subject section, only the email body

Once you have made any changes that you need, save the template and select it from the Email Templates screen

Click Save to exit Preferences and your default template will now be set for individual Invoice emails.