Unless you have opened an ebay return case and used the postage label supplied by ebay you have no proof your seller received the parcel.
You have proof of posting, That bit of paper should have a reference number which you can put into track and trace (assuming you used Royal Mail) If that tracking number shows as delivered you could open a return case now. The tracking number will be wrong because it will be for the ebay label so you need to change it. To do this you will have to ask ebay Cs tomorrow morning after 9am.
Once you've added the tracking number the delivery status should change to delivered and if you don't get a refund within 48 hours you can escalate your case.
