I had two orders where I had entered the details for sale to the US, and applied the postage. Both items were sent International Tracked and attracted US customs of £3.20 including the 50p handling fee.

However, after printing the labels, the customers decided to change their order, so I asked for a refund "Order placed by mistake".

I received the refund in due course, but NOT the US customs charge or handling fee.

There is no reason for that - Royal Mail should know that the packages were not posted and therefore not pay out the US customs or handling fee.