Can anybody help?

A customer has incorrectly customised their product and we have agreed to send a replacement free of charge except the postage costs. There is no way to do this in Amazon seller central, l have been advised to ask the customer to order and then we refund them for the product. Amazon will still charge fees on the total cost even though we have only received payment for the postage, meaning it will cost us more than we take.
Any suggestions please?