I would suggest finding out the vibe of the store/chain and then dressing up a bit more than that. Overdress a bit because that would be better than not dressing up enough. And smile!! Good luck!!
I would throw on a jean jacket so you are covered up a bit but not too stuffy, and then some cute wedges or esparilles! They want to see your fashion sense and this is the time to show it. If you can't dress yourself, why would they hire you to sell to other people! Since I am sure that you can dress we just need to find something perfect, so they hire you on the spot.
Ok I fully, fully believe in wearing their clothes on an interview, # 1. to me was whether or not your like the clothes! If you are going to sell them, then you should own some. If you don't own them can you go to another location and buy some? # 2. I decided whether I was going to hire them in the first 5 minutes, by how they were dressed, how they shook my hand, and other small things. Not too sound too harsh, but I am giving you the truth.
Now I realized that I sound like my old mgr. self, which I try to forget sometimes, so please do excuse me.
On the accessories, these were a great start for me to make the person feel at ease, so I love being able to compliment them on their accessories. You may want to wear some other accessories to show your true fashion sense that is in their style. Just so you don't look too cookie cutter-ish!
Now some may disagree that you need to look more professional, but I don't agree within this industry and by looking at the site, the all about is section showed the girls casual, so I would go with that. No jeans though.
Good luck and let me know if I can help at all in any way!