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 the return e-mail message for each store.
Head to your Account Settings (the wrench icon in the upper right).
Next, select Selling Channels and then Store Setup from the sidebar on the left.
Click on Edit under the Actions column for the store's return email message you'd like to edit.
Click on the Returns tab, and scroll down to Return Email Message to compose your message.
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.