As a Private seller your fees are actually slightly less at 12.8 % plus 30p per transaction.
Before it was a 10% seller fee via a monthly invoice to eBay and 2.9%plus 30p paid immediately to PayPal when an item sold.
May I politely ask why there is a problem 'predicting' a real cost of postage? I see you sold a large and very lovely sound system. You could have weighed all the parts separately, added them together and got an accurate postage for the weight and size of your item from Royal Mail or the courier used, no prediction needed.
As an aside you sold that item twice. Did you cancel the order citing Buyer did not pay? If not you have also paid 2 lots of seller fees, as eBay will not know a bueyr has not paid unless you tell them.
If your buyer has not paid within 4 days, you can now cancel that order stating buyer has not paid, and your seller fee will be refunded, the item will be relisted unless you choose it not to relist.
This link will explain the procedure fully.
https://www.ebay.co.uk/help/selling/getting-paid/resolving-unpaid-items-buyers?id=4137&st=3&pos=1&qu...
