I hate to say this, but....
First off, DON'T TOUCH MY CAR.
Second, DON'T PUT GARBAGE ON MY CAR.
Because now that you've left garbage on my car, I have to decide whether to discard it in the parking lot (for them not preventing it and it being their problem imo) or I have to find someplace else to discard it.
Do you know how my wipers work? Where exactly are you planning to put this garbage on my car at? If you damage my car while screwing with it, I'll hold you accountable for it. If I catch you doing it, you have an entirely different problem that I might make you deal with immediately. I'm not violent, but I don't think you want to have that conversation with me.
Do I read shit people put on my car? No. I look around the parking lot in case I can give them their garbage back directly. Would I email you If I looked? Probably not, and I'd probably respect you for signing it.
Issue #2 Does it help Paul? Maybe.
I would not be afraid to flyer a courthouse parking garage with FIJA material. But in that case, I'm also willing to accept the sentiments and results I listed above from someone like me. I think you have a whole ton of gray area to think about.