I wouldn't mind betting that the buyer did it on purpose, hoping to avoid a postage cost.

If I hit reply to a PayPal notification, I don't see the buyers email address, as Dave suggested. For me to see it, I need to right click on the buyers name, and then hit add to contacts. That then shows the email address. Of course I don't save it!

If you didn't get a notification, go into PayPal to the transaction, and select to refund. You used to be able to see the address where the money was being refunded to. If that is still the case, then send an invoice that way.