In the FAQ:
What types of material can I list as an Amazon Short?
Any previously unpublished short-form work (2,000 - 10,000 words, fiction or nonfiction) you've created that your readers would find interesting. An Amazon Short could be a single short story, an update on a well-loved character, a compelling speech, additional material that enriches your published works, or even your commentary on your work or other subjects. Some authors have chosen to treat this as a "laboratory" for experimentation with new genres, themes, etc. We are open to creative ideas for new work.
This will drive collection development librarians nuts...