This is an old, old game. While the "advertisement" of the donation (license plate frame, windshield sticker, bumper sticker, or membership card) can vary, it's been around a LONG time, and it simply works.
I've seen it myself. I was with a buddy in his Corvette in Pierce county, WA, and he was driving 60+ MPH over the limit and got pulled over. I figured he was going to receive a massive ticket, if not jail time, but I saw the cop glance at his sticker (in the corner of the rear left passenger window) adverting a donation to the local police union's "old cops home". He got a not-all-that-stern lecture, and that's it. He was going roughly 100 MPH in a 35 zone!