Seeing the stunned look on said shopkeeper is all the answer you needed. "You should close your mouth, you know, or you might catch some flies" you said, with a big flirtatious smile, before asking "And now, what the next step?" still flirting.
While still gobsmacked by how fast you learned the art of shape-shifting, and quite embarrassed by how you were acting, the shopkeeper still notice your double-entendre. "Not what you're thinking of doing," she say, gently pushing you away.
"First, you need to know that succubi don't simply act as their disguise, they become their disguise. What I mean by that is… You will naturally act like the persona you created for your disguise rather than how your true-self would. Wh-which is probably why you're trying to seduce me. Your disguise make you see me as weak willed, I do have a soul after all, which make her see me as prey."
"Oh, trust me, if my current self wanted to add you to the menu, we would already having sex," You answer. "I'm genuinely interested in you, sexy. I'm simply using this disguise natural inhibition to make it clear, my real self is too shy to dare. Be a dear and try to contact me once you escaped the clutches of your dreaded ‘mistress’, would you?"
Kissing her on the cheek, you finally take your distance and watch her blush. Satisfied with the effect you have on her, you finally help her remember your question. "So, now what's the next step? Or is there more I need to know?"
"Yes, there is more you need to know. First, as a succubus, and like any demon out there, you are now a bi-sexual nudist. No compulsion to go around naked, but you would happily let the world admire your body in its full glory if you safely could."
"Secondly," she continue "be careful when you find someone to your taste. First because while succubi can control their hormones, you might accidentally release yours at the beginning of your new life, especially when seeing someone you would be happy to fuck, and said hormones are extremely powerful against humans, to the point you can make gay guys go straight for you, straight girls go lesbian for you, and asexual peoples develop an interest in having sex if its with you."
"Thirdly, most people see succubi and incubi as either sex or lust demons, if not both. The truth is, those are only part of our ‘domain’. We are demons of longing and pleasure. All kind of longing, which include lust, and all kind of pleasure, which include sexual pleasure. Which mean as long as people around you, including yourself, feel some kind of craving or pleasure, they feed you. Of course whats the most sustaining, and the most delicious, is lust and carnal pleasure. What I'm trying to say is, never hesitate to please yourself in all kind of way, whatever form you took. If you don't you might end up starving, which might make you jump the bone of the nearest person until you kill them on the task, and maybe take their soul while at it."
"Lastly, when disguised as something else than a succubus, you need the kind of food needed by what you're disguised as on top as what succubi feed on."
She finally said all she had to on the subject. "Now all you need to do is go outside of this shop and, as soon as you're out of sight of the shop, go back to your real self. Then, this magic-mall should make naturally guide you toward where you need to be, probably a magical clothing store, to finalize your transformation into the first human-like succubus."
Before letting you go, she come near you and kiss you lightly on the lips. "My name's Gwenaëlle, by the way."
During the short time you were back in the shop, as you walked toward the exit, still disguised so the ‘mistress’ won't find out about how special you are, you looked back in her direction for a short time, just long enough to loudly and say "Sad how the shop policies won't let you go on a date with me, hot stuff!" Then got out.
— – - – —
Unknown to you, once you are outside the so called ‘mistress’ approach Gwenaëlle, who is back in her human disguise, and ask her. "Does she know I made the costume so, once she ended her transformation, she will come back and become my slave?"
While Gwen's face show a sad but resigned look, internally she is smirking. Nobody can control a royal demon after all, not even with the mistress's illegal modifications on the shop's costumes. Better even, the mix of said modification with demonic royalty will give all the succubi slaves of the mistress to the new princess, maybe even some of her other demonic slaves. Slaves who you will free, she is sure.
But for now, she needed to act as if nothing had changed. "Of course not," Gwen assured the mistress "I can't risk you being found out. …Not fault of wanting to…"
"Good." Calmly stated the mysterious mistress. "I found someone unnaturally resistant to not only my magic, but the mall's too. As thanks for your good work, I might give them to you tonight, as your next meal."
"You're too generous, mistress…"