I would like for the Featured Image to appear at the top of the post page itself. But not for all posts. I assume the easy way to do this is with a shortcode? Is there such a shortcode? Or is this already built into the Plus version of Septera?
If you wish to display an image (be it the featured image or any other) on specific single posts, you can include the image directly at the beginning of the posts themselves.
You might be able to find a plugin which provides the functionality of displaying the featured image via a shortcode, but it sounds to me a more complex solution than simply inserting the images in the content.
Repository themes are not allowed by the guidelines to include shortcode functionality.
There is no Septera Plus theme yet.