Dark and Light at present is hosted by Snail Games USA of which on the main games website when clicking the buy now link, you will be sent to the steam page for Dark and Light to purchase the game at £13.99. Now this could change depending upon a sale of which it tends to go on quite a bit and with recent news around the mobile version there may be more sales to come.
Dark and Light - Shard of Faith is a Free DLC (Downloadable Content) for Dark and Light of which when you download the original base game you automatically get this included in the game.