You can find the Add-On Services option under the Wash Menu section on the sidebar, click on it and then click on the “Create Add-On Service” button located to the right.
There are 3 options: Fixed, Stackable and Variable. Select Fixed if you want a set price. Stackable if the service can be added multiple times per wash. And Variable if you want your employees to enter the price each time the service is added, this option is recommended when the cost of the service may vary depending on the size of the vehicle, for example.