You need to register with PayPal, check in the PayPal invoice that it says Free returns on us. On the PP website, not app.
If not registered and already purchased, may not be able to claim.
If not purchased, you can register here
https://www.paypal.com/uk/webapps/mpp/refunded-returns
If all good, you sort the return through SP and then apply through PayPal
Take a photo of the parcel with the returns label affixed as you’ll need, plus a screen shot of the payment for the label on SP (I just use PayPal to pay).
It probably sounds as complicated as it is (actually it’s ok once done a few times!)