I was interviewing for staff earlier this week.
Main tips are to be smart and friendly (the kind of person they'd like to work with), not overly formal so you come across a bit strange (unless it's that kind of place). If the staff dress in a certain way, use that for guidance.
Don't just give short answers but don't ramble, if the conversation goes a bit quiet then ask a sensible question.
Be early and sit and smile and look interested.
If you're nervous or likely to fidget then take a copy of your CV in a smart folder and hold it on your lap.
When asked about skills give examples of when you've used them to good effect without coming across arrogant.
It's likely you have the kind of skills and experience they need hence why they called you to interview, they will also be looking for someone who's nice to work around and fits in with the firm (check out the kind of people that work there, how they dress etc...) and their customers.
Might sounds like a strange final point but wear nice "jewellery" which is not a million miles from what they sell eg if it's a upmarket pricey place, wear a smart watch, cufflinks etc...
Be interested about the job, especially as it's an independant place. Ask about the kind of customers they have, what the job entails, who you would be working and under.
A smile is a good way to react without going over the top to show it's something you would be happy doing.