Originally created in 1934 by the U.S. Postal Service for the U.S. Fish & Wildlife Service, duck stamps are federal licenses that hunters over the age of 16 must buy in order to hunt migratory waterfowl. Even though stamps can be bought at a variety of locations, including sporting goods stores and online, they ultimately arrive by mail. Electronic stamps remain valid for up to 45 days while people wait for the actual stamp.