In More > Setting > Invoice, you could set
1. Invoice number pattern
2. Invoice Template
You could upload the logo, header and footer to be displayed on the invoice. Clicking on Preview will show you how it look like before Save.
3. Invoice Email Template
Input the subject, then input the content in the Text box. Click Show More to input placeholders in your emails.
4. Select Contact Method below and save.
Enabled Auto Send, invoice will be sent to the customer every time when it is updated.