Does anybody know if it is legal to charge a commission on a service which is not provided by ebay?

I charge a buyer exact postage according to Post Office quote (not ebay postage),  buyer pays me exact postage, I take item to Australia Post Office, pay that same exact postage to Australia Post,  the 100% of the postage.  ebay then charges Seller 9.9% for a service they are not involved with:  the postage is between Seller, Buyer, and Australia Post.

It's like imposing a kind of g.s.t. on an item or service that ebay does not provide and are not involved with.

Paypal collects the postage money with payment for the seller,  ebay does not collect any money.

All ebay collects is a seller's listing and final value fee, nothing more in my case.

I would like to know if anyone has any connections to find out.

I am not whinging about it being unfair,  a seller is meant to absorb selling costs and not add more than correct postage fee on a buyer.   

I am curious to know if it is legal.