The return cost depends on the weight and size of the product you want to return, as well as the return method you choose.
You can find more information about different shipping methods at Deutsche Post's here: https://www.deutschepost.de/en/b/brief_postkarte.html. If you want to send your return with Postnord, you can also buy postage for the selected shipping method directly on the website or in the app: https://www.deutschepost.de/de/o/online-frankieren.html.