To set your default invoice comments, head over to your Account Settings and look for Default Invoice Text under the Configurations tab.
There you can include any text that you want displayed on your invoices, such as "Thank you for your business!" or a note about your return policy.
When a new invoice is created, that message will automatically show up under the Invoice Comments section. You can add to or remove this message from a given invoice.
Invoice comments can be found at the bottom of the invoice, directly under the balance information.