How to Approach a Friend About Paying You?

So this summer I taught a little art class in my home twice a week. I charged a reasonable fee for the students plus a fee for supplies which I then purchased for them and they kept after the class was over. I also sell prints of my own artwork. My friend and neighbor said she would like one print and I said I would give her half off because she is a friend. I went ahead and delivered it to her; she didnt pay me right then but said she would when she had some cash. Then she signed up her two daughters for my art class, so I purchased supplies for them both. One of the daughters decided not to take the class. So, I was going to charge my friend for that daughters supplies only if she still wanted them, otherwise I would use them for myself. I asked parents to try to pay in full before the lass began but said that friends could pay me as soon as they could. This friend told me that they were unable to pay me at the beginning because they had some other debts they had to pay off and I said it was fine,I would just let her know how much they needed to pay after the course was over. I sent her an email letting her know how much she owed for the class,supplies,and also that art print she still hadn't paid me for. A week goes by, I see her a couple of times and she never brings up my email,never tries to get ahold of me about paying me or anything. I sent her the email again,giving her the benefit of the doubt that maybe she never got the first one. Another week or so has gone by and I still haven't heard from her. I am not sure how to approach her about paying me. it's not like she owes me a ton but it makes me feel like she doesn't value me when she blows me off like this. And although I was casual about the whole thing, teaching and selling art is my job that I do for myself and I expect to actually be paid for it. I don't want to approach her in a way that seems rude but I'm not sure what to say? What would you do? Am I being unreasonable?