If the Unpaid case is still open, state in it that no shipping cost has been given. Or pay the shipping that is given, even if it is the US domestic rate, since that will show that you are willing to pay.
If it is closed, the transaction is over. While you might win an appeal, it would make more sense just to move on. One UID Strike will not hurt you much, but do make sure in future that you have a firm quote on shipping BEFORE bidding, since a second strike will make it very difficult for you to bid on eBay.
Once the seller has chosen an Unpaid Item Dispute, he cannot switch to a Mutual Cancellation. (And vice versa).