I put fliers all last summer in mailboxes had no problems with the Post Master. Either way; not legally speaking but in terms of the effect, what do you think is better for an idea?
I think on the cars because if its in the mail box they might just throw it out but if it was on the car they will take it off and stick in the car along with all there other stuff and one day get fed up with the mess and then find your flyer and give you a call. :nixweiss