Never haggle with the price. See rule Five. If you use them again they will overcharge you and then reduce it when you haggle. You don't want somebody working for you who feels like they are being cheated. Don't promise to get them a lot of work with other people.
Contractors hear that all day long from people who want a break on the bill.
Don't give the contractor an opportunity to laugh at you. Look at the truck and the service person. Pay by credit card if possible or pay by check when the job is complete. Make sure you have a contract though. I have been doing business with Miller's Mechanical for over 6 years.