I have a question as to "industry standards" or "best practices"

I met this contractor at the Hynes Convention Center Home Show, and got him to install sidelights on either side of my front door. I got a written Estimate. Then they came, and said that there would be more labor involved than they thought.

I then emailed 2X, and spoke by phone to the Owner, asking for an amended Estimate. He hemmed and hawed, and said that they wouldn't know, but that it would not be that significant.

They came and did the job. I now have an Invoice for more than 3 times, almost 4 times the original estimate. I now (my bad) also find that he is not licensed or registered in my state.

I need to know how at approach this Contractor. He gave me a Labor breakdown to show his costs, but I would not have gone forward with this job had I been made aware of this type of cost overrun.

The many Agencies I have contacted only tell me to file a report or take him to small claims. Before such a decision is made, I want to try to contact him to come to an agreement on a reduced bill (perhaps 2X the Estimate instead of 3 or 4?) I don't think that starting the letter or phone call with "I am going to report you" will be helpful to my case.

So I need some wise opinion as to what is a fair offer on my part, and I would like some information on "industry standards" or something to that effect.