When any payment method is used like card/PayPal we pay a transaction fee to the merchant which is 2-4% depending on the payment method.
This is nonrefundable as they charge us for processing the payment. It's true for everywhere you pay with your card or PayPal or klarna
Now when we refund the money because of our mistake we always refund in full as it's not the customer's fault.
If items are shipped and then cancelled then shipping needs to be paid as well