Click ADMIN > SERVICES > SERVICE ITEMS > SERVICE ATTRIBUTES from the menu.
If you do not want a service to be charged to your clients you can select this option. For example, if you provide day care for a dog at a fixed price, you may want to schedule a staff message about taking the dog out for a short walk in the diary, but you may just want the fixed price daycare service to appear on the client's invoice. In this case, you would make the message service item non-chargeable.
How do I make a service non-chargeable?
Check the "Non Chargeable" box and click "Update".