You can add Add-on items in Localisation Management > Add-on Items. 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.