You can customize your invoices and add content to the appendix such as terms and conditions, refund policy, return policy etc.
To do so, visit your Settings page and choose the "Invoice Settings" card. Click the Invoice Customizer, which is where you can change the look&feel of your invoices:
Click on "Edit Content" to edit the Appendix title and information.
Hit save and the content will be included in all your future invoices.