Not to over-analyze a comic, but the business model might not be awful in this case. They sell all potions for 20gp kind of like the original dollar stores. The one particular item is at a loss, but the majority may be at a profit or at least the aggregate based on volume is a profit. Basically they have loss-leaders to get people to use them as their only store.

This may seem like a ridiculous strawman, possibly because it's presented as a comic strip, but I've seen similar conversations on the TV show Kitchen Nightmares. Possibly worse actually, as the business owners didn't even know how much it cost them to make their most popular dishes.