The old man that runs the medicinal shop has a relatives that live in Riyuuto Village. Because Naofumi saved many people during the second wave, the old man happily assist Naofumi whenever he's needed.
The owner of the medicinal shop gives Iwatani Naofumi a book with many recipes for mid-level medicines to help him learn how to craft better quality medicine as thanks for helping his relatives during the second wave. He then tells Naofumi that the Magic Shop Owner has asked for him to stop by when he has the chance.
The owner of the medicinal shop returns as Iwatani Naofumi's personal tutor. He is part of an Apothecary Guild which was constantly fighting with the church over treatment methods. There are very few skilled people among either groups.