Thanks guys
^^This^^
Garage door glass is flat, completely different than automotive. You won't learn much other than waste film.
There is some good information is this thread. Baby shampoo is the way to go. I would recommend shrinking ALL the windows, not just the rear. I would also recommend a dry shrink in the back and wet shrink on the sides, otherwise you will get fingers when applying it. I don't use the precut stuff. All the windows that I do are custom cut to "your" windows. The precut don't always fit perfect and you get light gaps.
$13 for film, I wouldn't trust that it won't turn purple, peel, fade, and bubble. I use only high end films, Suntek, 3M, Madico, and Solargard. You get what pay for. If you decide to go the professional route, I charge $220 for wagons, shoot me a PM and we can set up an appointment.