You need to open a dispute via paypal (if that is how you paid) and then escalate to a claim and paypal will give you the address to return it too via trackable post at your cost.
Paypal will ask you to enter the tracking details once posted and when it is received by the seller you will get the amount you paid to the seller in full as a refund.
If you just return to the seller without tracking and without going via a dispute you are at risk of not getting a refund if the seller claims item was not returned.
