ausador,
What a story. I'm afraid it happens too often. Unfortunately, there is a stigma against the trades caused by a few bad apples and by the general public's ignorance of what it costs to run a company. Many people think they are hiring you for the day/week/month just to cover your salary.
The truth is, you never know whose house/business you are walking into when taking a service call. The only way to protect oneself is to have everything documented and signed as you have.
At least it had a happy ending.
