% eBay takes for each sold item ??

Just want to ask if anyone else is noticing that when eBay takes their fee for items sold they take between 11%, 12% and sometimes just over 12% and they are taking a percentage of the postage as well.  I thought that their fees could not be changed and it was a set fee, but the fees taken are different quite often, and I didn't realize they could take a percentage of the postage when that money belongs to Aust Post (which is who I use).  Just a bit confused, and feel they are doing the wrong thing, but I don't think they can be stopped 🙂