post https://app.shuttleglobal.com/c/api/instances/{instance_key}/payments//refund
You can refund a payment in part or full, or simply mark a payment as refunded (if refunded offline). You can refund "PAYMENT" and "CAPTURE" type transactions, but not "AUTH" (you must void an AUTH that has not been captured).
Note, support for multi-refund (e.g., I pay $100 and refund $20 followed by a second refund of $80) is not available on all gateways.