Kept mine in the car for years too and thought it was fine until the summer heat warped all the pages and made the glue on the sticky ones melt into a solid brick. Leaving coupons in a trunk is just asking for trouble, rain or shine, hot or cold. That binder was never meant to live in a car, it is paper and paper does not survive there. Honestly you got lucky the manager was nice about it, most places would just shrug and say sorry. Move the binder inside, keep a small envelope of just the ones you need for that trip, problem solved.