It depends on what you are selling.

Do your items fit into any of the eBay flat rate satchels or boxes? If they do that is often the cheapest way to post and you can prepare your parcels before you get to the PO. You buy the packaging in advance from Aus Post and print the label through eBay. The postage cost is added to your monthly invoice.

More info

http://auspost.com.au/parcels-mail/terms-and-conditions-ebay-label-printing.html

http://sellercentre.ebay.com.au/postage-and-shipping/australia-post-labels

If your items don't fit into the flat rate satchels you can still use the eBay Postage labels and you receive a discount off the normal over the counter prices. You need to use your own packaging and enter the parcel weight and size dimensions to print the label. The postage cost is added to your monthly invoice.