Your post doesn't really make sense because there's no eBay charging system where you pay a flat rate of 85p per sale (other, posibly, than if you're only selling items woth £8.50).
Does eBay see you as a private seller or as a business seller? There are different fee structures for each. See here
https://www.ebay.co.uk/help/selling/fees-credits-invoices/selling-fees?id=4364
and here
https://www.ebay.co.uk/help/selling/fees-credits-invoices/fees-business-sellers?id=4122
(If you can explain whee you think the 85p figure comes from, we might be able to spot where any additional fees are bein racked up).
One more thought:
Are you absolutely sure that the email you've received actually comes from eBay? It might well be a spoof email, trying to trick you into paying money into a fraudster's acount. Do NOT click on any links in the email. Instead, go directly to your eBay account and look at the information there. If you genuinely owe the money, there should be a detailed breakdown of how the total was arrived at. If there's nothing there to indicate that you owe the amount, simply delete the email.