You'll get the address once your buyer has paid.
The correct address to send the item out to is always the one showing in the order details in the Awaiting Dispatch section, here:
https://www.ebay.co.uk/sh/ord/?filter=status:AWAITING_SHIPMENT
Or, if you're buying a postage label via eBay it will automatically pick up this address.
If you're writing out the delivery address on the package you must include any code that's in the address, it identifies the buyer.