The first thing in every job is to obtain a quote. It outlines the completed task or how much it would cost to accomplish. Your HVAC quotation should outline what the customer should expect, and you should not begin work unless they sign off on it. An invoice, on the other hand, is the final bill you give to the client for the work you have accomplished. This is the project’s final step. The cost on your HVAC invoice may vary from the price on the quote based on how much the work has increased during the project.