Mark an invoice down to $0 (free) by using a 100% discount code.
To create your COMP discount code:
Navigate to Setup > Company > Discount Codes
Click the +Add button
Complete the form for the discount, with the following sttings:
Percent discount: 100
ON: Apply to detail packages
ON: Apply to extra services
ON: Apply to mobile surcharges
ON: Apply to manual entries
Click the Save Discount button
