Click on the receipt. Just above the dollar amount it will say in blue letters "Cancel a Transaction". Click on that and follow the instructions. That only cancels the sale. If they paid by PayPal you need to also go to the PayPal site to refund their money, or refund it through whatever payment process they used. Besure to send them both an email and a convo (just to be safe) that you cancelled their sale, refunded their money, and why. You might want to mention that you couldn't wait any longer as you are on vacation and your shop will be closed for the rest of the month. Also, if they still want the item, you would be more than happy to sell it to them when you returned.