Legal Question? It's kind of long, but I really need to know what to do :(?

O.K, so this is kind of a long story so I'm going to shorten it as much as possible. In January of 2011, I started babysitting for a friend of my best friend. The agreement was that she would pay me $40 for each day I babysat. After a while she stopped paying me promising that she would pay me later because something had came up. She owed me for two weeks before she went to Panama City and the two weeks after. So at $40 a day for 7 days would be $280 and $280 per week for the 4 weeks she didn't pay would be $1,120. At the end of the month my boyfriend encouraged me to stop babysitting for her which I did, she sent me messages cussing me out and accusing me of things I never did, and saying she doesn't owe me that much. I have babysat since I was 10 and have never cheated anyone. I told her that since I did quit without a notice to be fair I would take off the last week so all she would have to give me is $840, but she doesn't even want to do that. I told her that I had a Maurice's bill that I needed paid off so she gave me her debit card but I can't use the card because she won't give me the pin. So not only do I have a bill in my name that is now costing more because of late fees, I couldn't even take my mom to the hospital when she needed because I didn't have the gas to make it to where she lives because I would have been stranded. This girl has messed me up in so many ways and after all I've been through she still refuses to pay me and cusses at me when I ask for a little money. I made sure to provide her and her kids with everything they needed I would take her kids to the movies, out to eat, and to the fair all with MY money. She never gave me money to take her kids places, it all came out of my pocket. There were even times I let her use my car to drive to work cause she didn't have a car and she would never put gas in it. I dropped two of my college courses so I could accommodate her work schedule, and after a while she took advantage of me and started making me come an hour to two hours before she had to go to work and would stay close to 4 hours at the place she worked after they had closed down to spend time with her boyfriend. So I was babysitting 7 days a week from 4p.m. to about 5a.m. for a whole month with no pay, I have tried to be fair about the situation but she doesn't want to be. I have thought about take her to small claims court but in the state of Alabama you have to be 19 to file a claim and I am only 18 and by the time I turn 19 it will be too late. My question is, I still have her debit card how legal would it be to go to the store and run her card as credit and buy something for the amount she owes me ($840) and return it to get cash back? I have the text message where she gave me permission to use her card to pay my Maurice's bill but seeing how I can't because I don't have the pin or any bank information to, would it be legal to do that? I'm just trying to figure out what to do because she messed everything up for me so badly and I just feel like I don't deserve what she is doing to me after all that I have done for her and for her children. I'm sorry this is so long, but I just wanted to share the important details, please tell me what I can do because I feel so trapped :(