It seems like only fanfics have it.
I only have found one transaction so far First Order. But the bag is gone now since I rebooted my phone after the app stopped working at roughly midnight Hongkong time. Now I have to read fanfics to get any more coins.
Now my other favorite stories have to be cut back while farming gold For Spirit Stones.
Why not simply make the add watching a alternative way to unlock Premium stories.
Give 1 Spirit Stones to unlock to chapter and watch an add every 250 words.
With the Stones you can get for check-in, voting and reading time bonus, you can make up 255 Spirit Stones a week.
Therefore you could then read up to 255 chapters per week or 36 per day.