In general I dislike the use of plastic bags. A very few are such beauties (like those from museumshops) they excape from this rule and are treated like treasures. No way I would consider to give them to somebody who suddenly needs one ;).
When F and M+L leave in a hurry to attend viollin-lesson, they use to grab an aweful plastic bag to carry the music books. You understand this should be ended. Here's a new bag for all the stuff, including the new ring binder. Because in the end I was disappointed about the width of the bag, I added two snap fasteners. They reshaped the bag fairly well, I think.