Why is my doctor charging me $500 extra for refinement?

I paid $5600 for my Invisalign(24 trays). I also did propel($500 extra), which cut my Invisalign treatment in half. Now, im on my last tray and teeth arent straight and im not happy with my result. My doctor is charging me $500 extra for refinement. I dont understand, why i have to pay extra for my refinement. I realize that the last couples of tray, on my bottom teeth arent even straight. How are my teeth supposed to move to the correct spot, if the trays aren't even straight? i am upset!