50% restocking fee on returned book?

Hi, I recently purchased a book thorough amazon for my summer course. I originally wanted to purchase the book + access code package. However, when I opened the package after it was delievered, It did not have an access code. ( I realized that there was a sentence with tiny grey font below the status of condition of the book that says: Access codes and supplements are not guaranteed with used items.) I returned the item immediately. But today, I saw that they took 50% restocking fee, which was $100.00. Is there anything I can do in order for me to receive that $100? I don't even know why they charged me $100.00 restocking fee.