If a customer wants to send back a device she's renting, she needs a return label to attach to the package and send it back.
If you have the customer's return label you can now easily send it to her by looking up the customer in the portal, scrolling down and clicking on Send return label.
When doing so you're asked to include the PDF of the return label. After you just have to click on "Send return label", confirm, and you're done! Like this:
Just always double-check you're including the right return label! There are address details of your customer on the return label after all..