You need to make the buyer whole right away. Either refund them in full, (you can use the reason "problem with buyers address"), or send the item out today with tracking.

Every day you delay brings you closer to getting a neg on your account.

Yes, Ebay promotes Sendle, but you are in no way forced to use them.

You would be better off signing up for an Aus Post "MyPost" business account, and use either your own packaging OR the new parcel bags which are NOT prepaid (get the ones where you need a label). You'll need to lodge over the counter,  but when the postal agent scans the QR code you get, you'll obtain a good discount, depending on your posting volume. I'm on Band 5, and I get approx $1.86 off the $9.30 parcel post label.