Does this shop say what brand of tint they use ? There are obviously different grades of film available. 3M has a good reputation. The shop I had my wife's car tinted and our son also used uses 3M tint and there's a lifetime warranty. I don't know if the warranty is on the film, the installation, or both though.
North Carolina law is 35% on all windows. The windshield can be tinted but only the top 5". That said, you can put 20%, 5%, etc on if you choose to. It's not illegal to sell tint that dark nor is it illegal for a shop to install it - it's on you. The law applies to the vehicle owner. 35% is nice though. Not too dark, but still gives a nice look and the benefits of tint. Couple other things..... The color of your interior will impact how dark the tint looks from the outside. The rear windows will look darker than the fronts even though they are the same % because of the giant windshield allowing so much light in.
