Note 1: This article refers to printing a return label that will be included in the customers order, and not to issuing a return label after shipping.
Note 2: This option is presently only supported for the Shippo integration. Please contact support to request this option for another shipper if needed.
First, create a normal shipping label. After printing the initial label (and before closing / refreshing the window), click the green button (Create Return Label) on the left side of the popup.
This will walk you back thru the label creation process, but a return label will be generated. This label is free until it is used. USPS labels expire at 365 days.