Is it too late to vote for the button-up shirt?  I dress a bit nicer than I would for the actual job when interviewing, but not too over-dressed.  I'm with everyone on the black pants and flats.

Michael's doesn't have sewing classes, but I think they have demonstrations.  You could ask if in-store demonstrations are provided by a vendor or if store employees are able to contribute to them.

I'm with humboldt_honey on this one. Button-up shirt, black pants and flats. Professional, but not overly dressed or distracting(loud prints, bright  colors, visible tattoos or body piercings). You want the interviewer to focus on you and the qualifications you bring to the job.

You've locked in the interview, which means they see you as a strong candidate for the job. The interview process means they want to get a better sense of who you are, qualifications, passion, and, simply, "is she someone who I, staff and customers would enjoy working well?"

Lastly, do your homework. Make sure you know Michael's, its products, services and classes inside and out.  This will impress your interviewer and will show him/her you that the learning curve will be short and that you will be ready to go once you are hired.