The seller can't ask for more money,only what the order details show.
The seller could cancel the transaction, which would automatically refund you,as you can't make a seller sell only leave the relevant feedback when they won't.Something along the lines of "Seller failed to honour the transaction" so it doesn't get removed.
If they don't cancel,then once the estimated delivery date passes,you can report item not received with ebay here,if you don't receive your item https://www.ebay.co.uk/help/buying/returns-items-not-received-refunds-buyers/get-help-item-hasn't-ar...
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