You can add Add-on items in LocalisationAdd-on's. Here you can add as many items as you would like your drivers to be able to add during the job completion screens. You can specify the following settings.

Name & Description - this will show on the drivers device and in the customers invoices. Item Cost - The cost for the item, this can be both a positive and negative. Item Cost Variable - Whether the driver can adjust the price field from the set cost. Item Quantity Variable - Whether the driver can specify increments of the cost field. Commission - Whether the driver should earn commission on this add-on item per their driver rates or not. If not, then the amount goes fully to the driver.