Those labels have a cost, and mean you can simply pack and post and don't get charged anything at the post office - the buyer still pays you the money you charge for postage, but then you need to purchase postage either from Australia Post directly, or (in this case) through eBay's postage label system.
Edited to add: if you have been charged multiple times for the same label / order, it sounds like multiple labels were created, you should have an option to cancel the extra ones that won't get used, I'm not sure what the time limit on that is anymore, it used to be you had to request to cancel within 5 days).
