I've got a bunch of personal music (and much of it purchased from Amazon) that I have organized playlists of. I want to use those playlists with the Echo Dot so they need to be in Amazon Music. I can export the .wpl files to .m3u. I can also export the songs in my playlist to a folder for uploading as a group but I can't see how to either turn the uploaded files into a playlist or (better) upload the .plw or .m3u files.
I have made some progress towards an answer, though not an exact answer to your question.
There is a "share music" function on both the cloud player and the downloaded Amazon music player. This is used by companies like https://soundiiz.com/ to extract a playlist from Amazon. Soundiiz allows you to download that data in several formats, but it does not have the name/location of the mp3 file (as far as I can tell that is not their fault, its not in the data available from Amazon) and so you cannot download an M3U file.
I have thought about writing a tool that will use the data in CSV from soundiiz or perhaps even just getting the same data from the Amazon export URL, and then finding the corresponding music files on disk and creating the M3U files... but I do not have time for all the things.
I seriously hope someone does this work, however, and it is very disappointing that Amazon traps the playlist in the Amazon platform by not just allowing the export of M3U files. I share your pain.