My Boss is Stealing My Sales

My Boss is Stealing My Sales: I am a young sales associate at a local retail store. I worked there all during college and even after graduating, I still enjoy the commission based set-up I have with the company. Even though I do not work full time there, I am still outperforming and surpassing most others in the business, including my boss. Recently I have discovered that she steals my sales so that she can list at least a few commissions on her tally. It is becoming obvious to me that she is blatantly stealing my commissions. How do I handle this?