belladonna, vervaine
the kindness of the shopkeeper has something reassuring and fits into what the apothecary itself has to offer: belladonna, which is stored with the verbena or the dragon intestines with the unicorn hooves, the hellbore and the briony - every ingredient has it's purpose and use depending on what it is combined with. and snape's liaison with the shop keeper is also not necessarily a poisonous or evil thing itself. in the right context it can be a healing ingredient.
what makes the story so beautiful is that neither does it insist in punishing snape (or his affair) for his infidelity nor does it attempt to make a point of harry finding out about it. there is no typical conclusion. whether harry might know or not know or choose to know without knowing is of no importance - very much like snape's decision to end the affair can have many reasons: remorse, fear of being caught or him losing interest - but in the end none of them are relevant.
i liked the unusual, but quiet modus operandi of this story very much.