Window Card Storage

I've been collecting window cards from shows I've attended or whose art I really like, but I don't have the space at home to frame/hang/display all of them. Since I'm sure I'm not the first to encounter this issue, how do you all deal with storage? Is it just a matter of storing them flat, or is there a type of album/artist's portfolio I should look into to keep them fresh and clean?

I have about 10 that i have gotten in the past 6 years. I have them in a window card bag and in the box that a frame for one of them was shipped in.