I find that anything plastic is not UV resistant enough. Sharpie will fade off in a matter of 2 weeks, and the plastic will break if moved from original position after a while.
I would advise getting metal labels and using a pen to indent the name of the plant into it. It will no rust for a long time, and it won't wear away either. There is a way to cut open soda cans to save money, which I think I will try this winter in all that spare time without an active garden.
Edges stay sharp!