Your seller shouldn't be adding taxes, selling fees or anything else to the item's cost AFTER she's sold it.
I run a shop on a world wide on line market place and the only thing I can think is that she's put a higher price in in case it gets lost (the insurance you mentioned). If that happened, she'd claim a higher price through the post office in USA than she actually charged you (a bit sneaky, not very ethical and possibly illegal but you've got the item so it doesn't apply to this transaction).
However, basically she's managed to get you an extra few pounds VAT/customs charge so she can make cash if it gets lost!
I would ask for her to refund that portion of the custom charge. She's probably happily doing this on postage insurance all across the USA but it won't make a difference to those customers as it's internal - she'll make cash if something goes missing as she's put a higher insurance price on it.
Her international customers, on the other hand, may get walloped with higher fees from customs as it either pushes them into the customs charge zone (in other countries) or, in the UK, adds 20p for every extra £1 she's added to the cost of the item.
Ignore the issue about she has fees to pay to her on line market place. You're not paying her extra for the item, the UK government won't give her any customs fees so the only reason for the higher cost is a 'sort of' insurance scam if something goes missing.
I have a small on line shop and use a particular on line market place to sell through. My costs of selling (fees etc) are added to my prices, just like everyone else on the site (if they have any sense). The market place I use is a US based one - it's usual, bog standard business to wrap all your 'costs to do business' up in a price. So cost of material, labour, fees, profit etc.
That's the price the customer pays and that's the price that goes on customs forms. You don't bump up the price on the customs forms - just like you don't lower the cost of the item on the customs form (if it goes missing the post office will only base a claim on the costs on the form - that's what she's put the higher cost on the form for). Very naughty and I'd ask her to refund the difference between what you should have paid for customs and what you were charged. She needs to learn not to do this.