The Invoice Preferences screen can be accessed from your dashboard by clicking on the ellipsis (⋮) on your Invoices App and selecting Preferences.
This will take you to the Invoice Preferences screen:
When an invoice is created, you are given the option to print or email it to the customer or lead it's been assigned to.
Email Notification Template - Email Template used when clicking the Email button on the invoice, which sends an email to the customer. To edit this template, go to Method:CRM's Send Email App, and select Edit Templates.
Print Template - When the Print button is clicked on the invoice, a PDF file will be created and will use the invoice template chosen here.
Note: When these dropdowns are blank, then the defaults are used.