Your session could take 1.5 to 2 hours with a few clothing and hairstyle changes. Take a look at their studio. Get all the details and your agreements in writing. Ask if they do in-store fashion shows or events where they use models. Tell them about yourself.
Models ought to get reimbursed in some way or other.
It is not always financial, depending on the store, designer, or the situation. Sometimes models get clothing in trade for their time and efforts and work. Sometimes you take a job for the experience and exposure, and to add some good photos to your portfolio. There are at least a few books on the market which I think might be helpful to check out. Women over a size 18 are still experiencing prejudice in the modeling field.