You should be OK. What PayPal would do normally is ask you to send it back to the address the seller has on their PayPal account, so as long as that's the same address you've sent the item to, it'll still work in your favour.
If it's been sent to a different address, that may invalidate you for Buyer Protection.
It would ONLY be ok if you sent it TRACKABLE, without a tracking number that proves delivery of the item back to the seller you may well lose any dispute.
Advice is voluntary. Kudos / Solution appreciated.
Just a follow up. Keep your receipt and claim your postage back through the free rerun shipping kink on the top right of your account (the truck) up to 15 quid and you can claim up to 12 times a year. Also if you didn't send it tracked PP can (if they are in a good mood) ask the seller to confirm that they got it back. BUT DON'T RELY ON THIS
