The Invoice Preferences screen can be accessed from your dashboard by clicking on the ellipsis (⋮) on your Invoices App and selecting Preferences.
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.