Go to the Resolution Centre and open an Unpaid Item Dispute. You can do this 48 hours after the listing ended.
Half the time the buyer will wake up, apologize, pay and leave positive feedback. You then ship and leave positive or no feedback as you prefer.
Half the time you will still be met with stoney silence. Close the Dispute as soon as possible (I think it is four days, but I haven't had a non-payer since the new rule). You will not be able to leave feedback, but you will get your selling fees back. The bidder will get a Strike that will make it harder for him to buy on eBay. he can't leave feedback either.
Add the bidder to your Blocked Bidder List.
Do NOT attempt to contact the underbidder until this transaction is completed. You may lose him, but even that is better than finding you have two payments for one item.
