You can complete an Invoice Refund Request if you discover there is an overpayment, a duplicate invoice was received, or the wrong party received the invoice.
You can do this in the Customer Portal.
1. Go to Dashboard
2. Go to Invoices
3. Click Invoice Refund Request
This is different than an Invoice Modification Request which is used if the details on your invoice are incorrect (email address, physical address, etc).
You will only need to fill out one of these, never both.