First, let's talk about mailing your own greeting cards. Some of them have the most adorable embellishments, and you don't want them to be squashed by the time they get to their destination. A simple way to help this greatly, is to write "Hand Cancel" on the front bottom of your envelope. This lets the post office know not to run the card through a machine, but to process it manually.
Now, a little bit about how I get my cards safely to you. I have read about plastic, and the baggies that many sellers use to send out their cards to protect them. Plastic can last in a landfill for up to 1,000 years. Yikes!
So I wrap my cards alternatively using my old magazines. I have found that magazine paper is extremely sturdy and protective. When I ship my cards out, I tear a page out of a magazine and carefully wrap the card in it. Then, I do it again just to be sure. Voila! A very protective wrapper for your greeting card, and green to boot.