Can you provide some pics so we can have a better idea of what you are dealing with?

Without seeing your predicament, you can try pressing them between using two flat, rigid objects and adding weight to the top.  You would want to keep them this way for an extended period of time, but check occasionally to see if they are flattening out.  If there's one curve, carefully reverse bending might be a possibility, but that could be problematic...

You could also consider heat pressing them, but I would be concerned about delamination, since I believe that the paper is glued to the cardboard on these.

I'm curious as to whether this is a common situation for these.  It shouldn't be, unless they were exposed to humidity?  Maybe the box was stored on its side?

Put between couple sheets of plywood with heavy weight on top sheet.

Or, pillowcase and hot iron. Don't leave in one place too long. (I've done this with big box PC Games, don't know how would work on what you have)