I havent tried one so I dont know any to suggest nor how I've experience it but in readers/users point of view I wouldn't mind having it on my wall but then again I'm a picky reader I might view it but I'll never read em fully so thats kind of my flaw but hey if its interesting I'll might read em. :) Hope it helps.

I won't go to an ad shop to advertise my fic. But I understand how tight the competition is - what with many fics around, you have a limited time in the latest updated section. Especially, if you write a fic on a popular band, like exo.

But if you want to try to advertise via a shop, choose one that is free and check the number of subbies that shop has. And like you said, others might not even check out the ad in their walls, simply because they don't like wall advertisements. If you are going to pay, I think it's best to save up and bid. If you lose, you get back your karma. If you win, all of AFF or most of AFF will see it and if it catches their interest, you get subbies.

No matter how much you market your story, it's the quality of your product that would make them sub. Cliche plus good writing works, but don't make it too common. Your story must have that extra something that makes it different. Updating regularly is also a way of advertising. If you put up updates regularly, say weekly, chances are high that people would notice. Other ways to advertise yourself is through contests. I know some find their reading material in contests because at least they are guaranteed that the entries would have been edited, with zero to low grammar and spelling mistakes.If the subbies in that contest like your entry, they might check your other stories as well. So even if you don't win, at least your name is out there. It's the same with review shops. If you get a high score, they would feature your fic.

I don't approve of them tbh :/