I can say with some certainty that you have something of value here, one of the long-lost correspondences sent to Christopher Robin by Pooh from somewhere beyond the looking glass. The postmark code clearly indicates a region of Oz accessible from an offshoot of the rabbit hole. Other than that, most of the message is in code except for several popular propaganda slogans such as "Death to Wicked Witches." I recommend the Antiques Roadshow or one of the new salvage/picker-oriented reality TV shows if you want to cash in.