Good day @Joow_1971,
Thank you for posting to the PayPal community.
This looks like an issue with your internal website(built via third party platform eCommerce) plugin/module.
To resolve the issue, I suggest to please contact your website developer or the support of your third party platform(eCommerce) to cross check your plugin/module setup and pass the valid API requests to PayPal system.
Merchants can perform Refund (NVP/SOAP) API :
https://developer.paypal.com/docs/archive/express-checkout/ht-basicRefund-curl-etc/
https://developer.paypal.com/docs/nvp-soap-api/refund-transaction-nvp/?mark=RefundTransaction%20#
For Refund (REST) API Integration, please refer the below guide links.
https://developer.paypal.com/docs/integration/direct/payments/refund-payments/
https://developer.paypal.com/docs/api/payments/v2/#captures_refund
Note : Merchant can also process manual refund, by follow the steps provided in the below guide link.
https://www.paypal.com/us/smarthelp/article/how-do-i-issue-a-full-or-partial-refund-faq780
Important : PayPal system is a payment gateway, which will receive the payment request payload (i.e Payment and Item details, Shipping details, Invoice number, Amount details etc.) from merchant websites(built via third party platforms eCommerce) and process the payments accordingly.
Merchants should handle their internal website code and pass the appropriate payment API calls to PayPal system.
Sincerely,
Chiranjeevi
PayPal/Braintree MTS
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