Common practice amongst contractors is to low bid and change order people to death. As a contractor something major would have to happen for me to charge extra and even then it needs to be discussed. And never sign off on liability. My business has a 5 star google rating. That is very important to the growth of my company. I would look into any place they look to get work. Ie angies list , home advisor or google. I also usually ask for a downpayment. People forget that there is as many flighty people as there is contractors.