You only need proof that you returned an item , otherwise all sellers would just say it's not received, and no refunds would ever be made.
You should have entered the tracking number into the case, and then escalated 48 hours after you posted the item back, and eBay would have forced the refund.
Open a case now with Paypal, as you are too late with eBay. Enter the tracking number and escalate after 2 days and before 20 to start the refund process.
