Problem Printing Invoices When Adding Job Info

72chevy4x4

I've figured out how to attribute materials purchased and time to a job and can add those figures when creating an invoice (or after the invoice is created by clicking 'Add Time/Costs'). The materials added now are line items on the invoice-I DON'T want them to show (bid by contract job, not a T&M job). How can I attribute the material cost but have them hidden on the printed invoice?