You can add Products to a Job in two different places - from the Master Job and from the Job instance on the Dispatch Board. Once you add Products to a Job, you can decide whether or not the Client will be Invoiced for them.
To Invoice Products from the Master Job, follow these steps:
From the Master Job, click on the Gear icon (located on the right side of the job line item). Then select "Add Product(s) to Service".
Choose the Product(s) applicable to the job. Also, ensure to enter the products fields as needed such as Quantity, Rate, Total, etc.
If you don't need to Invoice the Product, deselect Invoice in the Product Settings so the Products used are not billed.
If the Product is marked to Invoice, it will appear on the Invoice when the Job is completed and the Invoice is generated.