Don't listen to them, most tint sold in stores is of low quality. Which means its hard to work with, fades, changes colors, tears, scratches, bascially looks like shit - IMO. If you never tinted before, your first many attemps will come out shitty, its an art that takes time to learn.
You could have a pro tint your rear window and buy some extra from them to try yourself on the sides/winshield strip. Start with the windshield strip, if you can't do that, STOP and take it back to the tinter.
Check out
www.tintdude.com for more information