Bookwalker has been having a lot promos of this kind with the first volume certain manga series being available for free for two weeks lately.
I've also seen Boys over Flowers, Ascendance of the Bookworm and Demon Slayer have similar deals (I actually believe that three volumes of Ascendance of the Bookworm are still available for free until the 31st, but I'm not that familiar with the series).
You can read either the manga in browser without an account or create an account, add the volume to you cart and 'buy' them, then download them on the app.
Go check it out!
by saywhaaaaaaaaatt