Shopify Payments incurs costs to process refunds, so we do not reimburse the original transaction fee. This aligns with our existing Shopify Payments Terms of Service (section D.6). Additionally, Shopify won’t charge you an additional fee to process the refund as many payment processors do, though. For more information, feel free to check out our Shopify Help Doc.
I hope that clarifies things. Let me know if you need anything else.