Hi,
I am using NopCommerce and PayPal Standart in Sandbox. I get error "This invoice has already been paid. For more information, please contact the merchant." from PayPal, when I tried to buy item.
PayPal says ex "Order Number 5" and order leaves in pending state in NopConverce.
The problem can be that for some reason paypal isn't accepting the invoice numbers, which are being created as 1, 2, 3....
In other xxxxCommerce they solved it using 'PayPal IPN ID' as invoice number. Unfortunatly I am so beginner that I cannot find how to correct the problem.