I'm pretty sure you aren't the first person to this - I have, and won't be the last.

If you don't receive an email reply from them in the next couple of days, I would phone them.  I have found the buyers I have rung for various reasons, appreciated the call, but guess some won't.  If their phone nos aren't on their address details, you can get their contact details from eBay, or check white pages.

The couple of times I have done this, buyers have been great and sent the correct item on, and I have reimbursed them for the postage.  That way each buyer would recieve item quicker. You are very generous, offering a refund.

Others with more experience may have other suggestions for you - Good luck with it all!!!