That's actually too low.
We pay Final Value Fees on the customer's entire payment, not just on some parts of it.
This has been Standard Practice in the payment processing industry since the 1940s with the introduction of the the Diner's Club card.
The FVF should be charged on the selling price and shipping charge, as you say, plus any sales taxes the buyer paid.
Those sales taxes are collected and remitted by eBay for the seller (there are more than 100 different taxes in play here between Canada, the USA, Australia, the UK , and the EU, possibly more). The fee on the tax is a service fee.
The shipping fee doesn't stay with you, you use it to buy shipping labels, in this case from FedEx.
FedEx charges sales tax on the label they sell you.
They are charging the tax on their shipping service.
EBay is charging the tax on processing your customer's payment.
BTW, the seller also paid tax when they purchased the product they sold you.
And their supplier paid tax when they purchased the product from the wholesaler who paid tax when they purchased the product from the manufacturer.
And that leaves out duty on imported goods.
FWIW.
All those fees and taxes are deductible when you add your gross sales to your income tax next spring.
