Stamp Out Invasive Species Act This bill directs the U.S. Postal Service to issue and sell a Combating Invasive Species Semipostal Stamp. Proceeds from the sale of the stamp must be transferred in equal proportion to the Department of the Interior and the Department of Agriculture for combating invasive species. (Invasive species are nonnative species whose introduction poses a serious threat to natural ecosystems and causes detrimental economic damage to local communities.) The stamp must be made available to the public for two years.