(Please don't use a razor blade on your back window. If you cut one of the lines it can mess up your rear defrost. Let the tint shop do it before they retint. I paid 130 to have mine tinted 20% with a lifetime guarantee as long as I own the car. If I don't use cheap cleaner and turn them purple, the tint shop will replace tint if it ever bubbled. Had it on 3 years with no problems. It's worth letting the professionals do it! Find a guarantee on the tint and the work!)
I just got my windows done last week at 5% on the back doors and i paid $130 also so ya my best bet is that i would most diff. take it to have it done bc ive done tint on alot of cars and i just get so pissed when i do it bc i have very little patients and i hate starting things and getting have way done and being like man i should have payed someone to do this