Why not just be honest and say it was a pricing error?
Pffffffft.
From: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Subject: Your Order with Now Group UK - Update
Date: Mon, 30 Dec 2013 10:42:56 +0000
To:
Hi,
Thank you for your order.
Unfortunately we do not have the system you have ordered in stock any longer and have been told we will no longer be able to get hold of them by the manufacturer.
I’m very sorry for any inconvenience this may have caused.
Your payment has been refunded.
Best Regards
xxxxxxxxxxxxx
Now Group UK Ltd