Add-on charges can be used for situations when you almost always sell one item with another. Eg a treatment to add to a service. This auto-populates the invoice with these items, you can still delete the Add-on item from the invoice if need be.
1. To do this go to the Cards tab and the service you wish to create the add on charge too.
2. Select the Create add on charge button.
3. A selection field will open at the bottom of the screen.
4. Use the drop-down list to select the item you want to add on to the service you are updating.
5. Select Save.
When you invoice a customer you will see that the add-on is automatically included. It can be easily selected by clicking the red X under remove rule.