*sleuths* Ooh! So this is why I've been getting new comments on 'The Blue Door'! ;) I'm not following this fest, but I had to follow you back here and have a look, because I can handle just about any pairing if infidelity is involved - I think we have a theme in common there. :)
Anyway, this was really well crafted. I had to scroll back up numerous times and check the word count, because really? Only 5K? It felt so much longer - in a good way - because the universe was so complete, and the scenes each accomplished so much, emotionally. I'm really in awe of that compactness.
I loved the quietness of the affair, the way there is no real fanfare to it, no time wasted on explaining how it began or ended - that felt very real to me. It's just there, something Snape does as part of his weekly routine, like buying potions ingredients. Well, although, at the same time, it is more than that, isn't it? It's routine, but also a tiny sense of adventure he injects into that daily routine - something which makes complete sense for a character like Snape, IMO, who is used to so much more regular danger and risk in his life than he would have, postwar. I loved the ambiguity of that first visit we see - is it their first time together, or not? I think not, but there's the chance it is. Either way, I love the way you use language there (and elsewhere) to say so much in few words.
I also loved the way you did the sex here, with Harry and with the shopkeeper. It's so nicely understated, sort of glittering at the edges of the story but never taking over, since our attention should be directed elsewhere, really, at the emotion of it, not just the acts.
I also can't help but think that Harry knows, right? Or is Snape simply reading too much into the double entendres, because of his guilt? I like to think Harry knows, if only because he comes across as so young and naive here, but I like to think he's not, underneath it all. He's keeping up the facade Snape wants, or the "Harry" Snape thinks he knows - young and naive - but he is older and wiser than that now, and he knows exactly what Snape is up to. In that case, then, I love that he doesn't confront him - or rather, that his 'confrontations' are so muted and sneaky - the cut on the finger, showing up in the shop, and their last conversation here. Beautifully done!
Lastly - gaaah, I can ramble, can't I?! - I'm with Penny above in boggling at the hostility to infideliy fic! I write a fair bit of it myself and thus have seen many comments like the ones you have here, and one day maybe I'll do a meta/poll thing about it, but I honestly am always amazed at how many folks in fandom list infidelity as their #1 squick. I think it's the most wonderful device for getting into characters' heads and emotions, myself. To each their own, of course, but I also think the story has to be very good, and thus provoke a real knee-jerk response from readers' guts, to get so many jarring comments against the theme. In which case: it's a great compliment to the writer, really.