Most bid work is lump sum. If the job costs less than the price, the contractor keeps the difference. If it costs more, the contractor pays the difference.
Example: a specialty contractor bids one price to furnish and install all equipment in a spec section. A missed quantity comes out of that contractor's profit.
A lump sum price is only as good as the takeoff and the scope behind it. Count carefully and check your count against the spec. Define the scope in writing with inclusions and exclusions. Scope changes after award go through a change order.