Its good to hear about a great ebay seller for a change
I would definitely thank the seller and leave her a really positive feedback
I know what you mean about feeling bad for the seller since the first item went missing, but really you dont need to pay her- its a sellers responsibility to make things right with you the buyer - either by replacing the missing item or by giving a refund
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Depending on her country she may even be able to make a claim from the PO for the missing one anyways even if there is no tracking number and its not a very valuable item (you can if you are a UK seller).