Unfortunately, you can not save any movies or series and listen to it later offline.
But this would be on the train with the poor network coverage necessary.
Offline goes so far only in the Amazon own devices as far as I've found it.
It would be nice if you could save 1-2 movies or 1-5Serienfolgen offline and play, then there are 5 points for the APP.
Stable wifi is usually only at the hotel or at home and since then I have to play other devices.