The postage is on them, not you. They'll have to resolve this with you not being out of pocket, whether this is you sending it back at their expense or them just refunding you the moneys is just something they have to weigh up. They live and die by their ratings, so losing a dispute (which they will lose) isn't in their best interests.
I'd follow up and ask how they'd like to go ahead with the return postage, and if you still get crickets go to eBay. They'll probably just refund you the full amount to save losing again on postage, but you've always got that fallback of lodging a dispute and having eBay pay postage and charge it to the seller. You have all the power as a buyer.
...this is why it sucks to sell on eBay, just don't do it
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