Personally I see nothing wrong with you placing your fliers for errand services upon people's cars that are parked in a public area in the winshield area so long as you do not damage the car. This happens all the time in California.There may be issues if you place the information on cars that is not a public area where the charge could be a trespass claim. For example a gated area that you have to cross. But, from what you have written, if cars are on a public street or in a strip mall, you should be allowed to do what you want to do under your First Amendment Right of Freedom of Speech.