So perhaps you'd like a paper bag instead of a plastic one. Maybe this is because you like how the paper one has a flat bottom, or you prefer the handles. Or maybe this is because you are a ginormous hippie and you recycle paper, so you don't want to get a bag you can't recycle.
All of these are acceptable.
What is not acceptable is going "Could I have paper, please? I'm a conservationist" in a cutesy little voice that would have small pink bunnies searching for a garrotte with which to kill you.
Nobody cares! No one asked! Keep your damn sanctimony to yourself.