Yep, exactly the same cost:
When your item sells, you pay 10% of the final transaction value, including postage. We call this a final value fee. We cap final value fees so you'll never pay more than £250 for a single item.
So it doesn't matter where the money comes from, you pay the fee on it. So if you want to charge the right amount for postage, you need to add it to the fee. But it is also why in ebay you can select for price + p&p when you sort.