You can use Click & Send to process your parcels and print shipping labels. When lodged over the counter you will receive a lodgement receipt and this is accepted by PayPal as proof of shipment.

https://www.clickandsend.com.au/home/NISS_frm_home.php?sz_Client=AustraliaPost

You can sign up to C&S and link to your eBay account. Once an item is sold and paid for you can login to C&S and the shipping info is automatically imported. You can use either your own packaging and pay the standard shipping rates or use Flat Rate satchels or boxes.

From October you'll be able to process and print your shipping labels for any Flat Rate items directly in eBay.

http://sellercentre.ebay.com.au/news/australia-post-labels