You can do multiple labels in ebay as Jigs said.

In ebay label maker page you can use the Australia Parcel Post Parcel option which then directs you to enter your own dimensions and weigh - I often find this cheaper than click and send.

Another way to try is the 5kg flat rate parcel label in click and send if your dimensions are similar, or purchase a 5kg satchel at the PO which are $17.60.