Browsed through the submission info but didn’t see an answer to this:
Should all main files and watermark files be zipped in the same folder, or different folders?
Which part of the site? I mean : TF, GR, etc…
Audiojungle
No, you ZIP only main files and upload preview mp3 separately, not with ZIP.
Ah yes, it’s coming back to me know. Had I tried a ‘dry run’ I would have seen the light. Thank you!
Optional thumbnail - upload pure jpeg/png picture.
Audio preview - preview file with watermark, just mp3, not compressed.
Main Files - ZIP compressed files without watermark, in your case a track.
Yes, I know 
it’s answer, not question. That mess was because “compressed” there means the compression of audio bit rate, not of archive.