I would open an unpaid item case now - I've been mostly lucky, but have found that the case does at least get a response, if not immediate payment....
An open case puts a time limit on when you can close it as unpaid to get your final fees refunded, but I think it's a good few weeks and the seller has discretion to close earlier or give more time.
Personally, I'm happy to give more time than the ebay minimum, but my patience wears thin if there is no communication from the buyer at all - that's just rude!