I learned how to make this from this Craftster tutorial.
Actually , I've already made several bags over the school holidays but I thought of doing a variation with this one. Instead of including a pocket for the bag to fold into (as described in the tutorial), I added a strap with a snap fastener. When the bag is folded up and the snap in place, it looks just like a wallet and should easily fit into one's bag.
Isn't that neat? Why ask for plastic?