I have a complete set of ten (200 stamps on 20″ x 24″ sheets) of the National Parks issue (Scott 756-65). What’s the best way to store them flat at a reasonable cost without too much risk of damage? My choices I’m considering are (1) mounting them in 22″ x 28″ picture frames or (2) storing them in a poster binder with 24″ x 36″ pockets. The former is kind of expensive, the latter poses greater risk of damage. Any thoughts appreciated.