Fidelity Credit Service Fidelity Credit Service dishonesty Internet

I had a case of paying $409 to a doctor. Since the service was not satisfied, I did not pay for a long. I received a letter from Fidelity Credit Service. A few days later, they called and offered $350 to close the case. $350 was a reasonable price that the doctor should have asked me. So, paid by phone.
A month later the doctor office sent additional $52 which the office paid to Fidelity Credit Service  and $59 (409-350). I never heard that the office asked a charge ($52) and the the difference ($59). 
Please, help me to know that whether it is legal or not for the office send such letter after solving the problem with Fidelity Credit Service. Or, this may be Fidelity Credit Service's and other office's ways of charging to the customers. Thanks