You've given them plenty of time cancel the order now.
Seller Hub > Orders > Awaiting payment where you can cancel the transaction. You can cancel transactions directly by using the 'Buyer hasn't paid' reason in Seller Hub,you'll get seller fees refunded non payer will get a strike against their account.
It may be too late after 10 days for your underbidder if they have bought elsewhere however it's worth a try
