If you're refunding only the charges added to a Sales Order, e.g. postage, freight or labor, you must create a Credit Note for the Invoice the charge was included on and remove any products or other charges before completing it. 

To complete a credit for only the charge on a Sales Order:

1. In the Sales Order that includes the charge being refunded, go to the Invoices tab. 
2. Hover over the Action Cog icon of the Invoice that includes the charge. 
3. Select Credit.
4. A new Credit Note will be created, auto-populated with all products and charges that were included on the Invoice.
5. Use the bin icon on the far right column of each Credit Line to delete the product from the Parked Credit Note. 
6. Use the bin icon in the Charge section to delete any other charges that are not due to be credited, only the refunding charge should be displayed on the Credit Note.
7. Select Complete.

Completing a Credit Note for a charge or product that's been invoiced will not change the original Sales Invoice. The Credit Note will only counter the the affect of the original sale. 

Note: If you only need to credit a charge line, this can be done by removing all other lines on the raised credit note.