If you don't get the option for Credit notes or want to add extra options to select from for line item codes dropdown on invoices, you can add line item codes by doing the following:
Log into Access RDB as someone with Administrator rights.
From the Ribbon, click Options and then click Look Ups.
Click Invoice Line Item Codes then click New.
Enter your new line item code in Code (such as Credit Note) and enter a Description (such as Credit Note).
Click Save.
The Line Item Code should now be available to select on Invoices.