Is it ethical of my Orthodontist to charge $2.500 for Invisalign refinements?

I'm a full Invisalign patient. Have 19 trays and just finished wearing the last. Some teeth didn't track so I asked for refinements. My doctor said that they would cost $2.500. I told him that Invisalign makes them for free and that our contract doesn't mention those costs and he replied that he needs to cover his own costs. He said that every visit of me costs him over $200. I already paid off the original $4.950. What can I do?