We do everything we can to make sure your items are in stock when you checkout. We don't hold stock ourselves (this is one way we make it cheaper) so rely on our suppliers to tell us when products are available. Occasionally they get this wrong, and when they come to pick your item it's not available.
Unfortunately we currently can't offer substitutes, but it is something we will be working to introduce very soon.
If you receive a notification that an item is out of stock, we will automatically process a refund and the money will be back in your account within a couple of days.