Symptoms
It is not possible to create Standalone Refund using some custom payed method (based on php-plugin), the error message is displayed instead:
Standalone Refund is not supported for ... payment method
Cause
The php payment plugin used does not support standalone refunds, there is no RefundCredit method in the list of supported methods, e.g.:
"REQUEST : MYplugin GetInfo"
"RESPONSE : {"supported_methods":["GoModal","Sell","Auth","Capture","Refund","RefundPartial","Void","CheckStatus","Callback","GetConfig","ValidateConfig","TestConnection"],
Resolution
If you needs standalone refunds to be supported, contact plugin development team to implement RefundCredit method.