Note: Cost Estimates do not necessarily reflect the final cost of a project.  Any additional materials, services, time, shipping, etc. may cause a change in final cost comparative to the estimated cost. 

Bids are distinctly different from estimates.  A bid is a fixed quote to complete a job which does not vary based on actual materials used.  For example, deciding to add more plants to an order after a bid has been submitted would require a change order to reflect the difference in material and labor cost.