Whenever you create a return label for a shipment, you have the option of sending that customer the return label in an e-mail. If you want, you can customize that message for each store.
To do that, just go to the Accounts Settings gear in the app.
Click on Selling Channels from the left side menu.
Now, find the Selling Channel for which you want to edit your return e-mail message, and then click on Edit under the Actions column.
Go over to the Returns tab, scroll down to Return Policy to compose your message, and then click Save Changes.
Repeat the process as necessary for any of your other Selling Channels if you wish, and you will have added your own custom message to your return label e-mails.