Coingate: the fastest way to get a refund is to create a request for our payment provider. They will arrange for a refund: https://support.coingate.com/hc/en-us/requests/new.
MixPay: The same applies to this payment provider. The fastest way to get a refund is to contact their support team: https://help.mixpay.me/en/articles/6836092-how-to-contact-customer-service. Or just write to: firstname.lastname@example.org.