I've seen in several shops that do custom orders, once the order is made, that's it. If you already purchased the stuff to make the item, then refund a percentage. Make sure you calculate how much it takes for you to make your items so that you know exactly how much it costs you to make each item and how long it takes. Then when something like this happens, you will know how much, percentage to refund since you have already bought the goods. Just remember you are dealing in a world of people of click and buy and click and return when people find something shiny, better, and cheaper. Welcome to the 21st century.