Most of my clients are my female co-workers, who I consider more of sisters/mothers than clients. As I detailed their cars I noticed their tire pressure/threads were low. So I began checking all cars that I worked on just to give them a heads up. When I get a compressor I'll fill the tires up. Are there any other courtesy services that I can offer that does not require much knowledge of automobiles? I can clean them but I can't fix or diagnose them. I just want to do this to let them know I appreciate their business.