How much should I charge?

So I am eighteen years old, and a student at a large university. I met a woman who just opened up a boutique just off campus. She needed help bringing customers in and asked if I would be interested in making some extra money helping her promote her shop around campus. Of course I jumped at the opportunity, as I am a college student and don't have a ton of money. I'm going to be handing out fliers around campus, and the shop owner asked me how much I'd like to make for doing so. I was wondering what a good rate to charge her would be? Keep in mind this is on one of the largest college campuses in the country. I don't want to overcharge her, but I also tend to be too nice about these types of things and end up making way less than I should. So what would be a good rate to aim for here? (She's been talking about paying a rate per flier, instead of per hour, as it's easier to keep track of).