If necessary you can create a credit memo invoice in FCS. Follow these steps and you will be on your way.
Step 1: In your service settings select the "Auto add 'Other' line item to all invoices?". Fill out the fields accordingly and click update.
Note: Once you have created the credit memo invoice you will need to go back into service settings and deselect the "Auto add 'Other' line item to all invoices?" option.
Step 2: Create an invoice using the "Simple Invoice" feature. Add notes to your description for internal or customer purposes.
Congratulations! In a few simple steps you have created a credit memo invoice.