Well actually I don't see here any hints that Aqua worked for the brothel... According to other dojinshi from this author Aqua just likes to fuck Kazuma. As for how exactly she knows if Kazuma orders something from the succubus shop it's simple - no demon can pass through her holy protection. I guess she caught already one ore two succubi, got information from them and probably came to some kind of agreement with them regarding Kazuma.
@realgundam If she did then she was a waitress there. Succubi are not regular prostitutes they don't accept money as a payment they need energy from their clients and that means that only succubi are working in a "wet dream department".