Is it fair to charge someone when they buy something for you?

My roommate told me she bought me some protein bars to share but she didn't have to do that but she told me it will cost me and I have to pay her. She said she is going to start charging me since I been getting her stuff to share. She said if she shares with me I habe to share with her. She didn't have to do all that but she chose too.
Only $6 for a box. I said ok cool.