Ana here, from the Shopify support team. :)
Currently, creating return shipping labels isn't a feature that's built into Shopify, however, here are a few apps that you can look into that do provide this functionality:
I recommend looking into these to see if any of them would be a good fit for your business.
Hello I am developing an app using Easypost for this exact issue. With my app you will be able to email pre paid return shipping labels to your customers.
Email me at firstname.lastname@example.org if you are interested in trying our app
Thank you for sharing your feedback about your need for a return labels solution in Shopify. I am happy to share that we recently added the ability for merchants in the United States to create return labels for domestic orders and email them to their customers from the order page. There's no additional setup required and you get the same discounted shipping rates that are available through Shopify Shipping.
Learn how to create a return label for an order here.
You can also check out the Multi Carrier Shipping Label app, that would help you generate return shipping labels for all major carriers and send it to your customers.