I have a little experience with this (sorry for the late reply too).
It can be anything on the rear passenger and rear windows, but the front driver and passenger, and top 6 inches of the wind screen have to allow at least 50% light in. The thing to remember too with a lot of factory windows, is they already have a tint to them, no matter how small, so if they have a 90% tint, and you get a 50% film, and put it on, your windows are now to 40%, which is illegal.
Also ask to see the last time the gun was calibrated if they check your tint. And make sure it's sunny. A cloudy day will skew the results. I got popped cause I have 35% all the way around (except wind screen, anything that is a full cover on it is illegal). When the trooper tested my window, she said it was 5%, there's no way, otherwise I wouldn't be able to see out of the thing. I asked to see the calibration documentation on her meter, and she wouldn't show me. I ended up fighting the ticket and got out of it. Other wise it's $90 bucks per window. (so could be a total of $270 a pop if they want to be *&$# about it.
Also I have never heard about the sticker thing. I've had my past three cars professionally tinted, and never had a sticker put on the window between it and the glass.