If you are looking at the sold items order details eBay tells you what your net proceeds will be after all costs are deducted.
In other words it's eBay's confusing way of telling what you will have left with after they've taken their fees and you've bought a postage label.
On a browser there's a "see more details" link at the bottom of the order which shows a true breakdown of eBay fees and what's going to your bank account.
Unfortunately on the app the link isn't there so the order looks like eBay has charged you for postage.
To see your eBay selling transaction fees and payouts to your bank use a browser and look at this page in your accounts payments section.
https://www.ebay.co.uk/mes/transactionlist
