How to ask for money back politely?

I bought a friend a $50 e-cigarette online to help her quit. I showed her Friday and she told me to put it in her bag at school so I quickly did. I said it was $50 and she told me she would pay me monday. mONDAY i asked she said she would pay me...its been about a week. How can I say pay me back! I am such a pushover how can I also keep people from using me.