You can mark it as dispatched by downloading the postage label. After that the buyer will automatically get a refund for the unused label in 30 days. That is all you can do. You cannot prove the buyer has the item. And as there's no tracking the payment will be held for 14 days.
If the buyer hasn't collected yet you'd be safer cancelling/refunding the transaction and relisting as collection for the buyer to buy again. They then get a collection code from ebay to give to you to prove they have the item.