I have been a member various associations/companys in the past only to find the level of service go down and prices go up and in the end just paying for a sign outside the workshop.
I now refuse to paid out hard earned money to these people who are in business to try to tell me how to run my business.
I agree with Peter keep customers informed and just do what is asked for and stick to your quote.
In the last 20 years the only complaint I think I have had is where one of my mechanics left his dirty hand or foot print on the carpet.
In the end of the day I want to go to bed at night knowing I have done a fair job at a fair price..
The only way to clean up the garage trade to have a government run scheme with licensed mechanics..

Mark