Doctor's Office Practices

I called our pediatrician because I had received a bill for $15 for my daughter’s last well visit which included a physical that was needed for school. It was my understanding that well-visits were without co-payment because of the ACA. Her pediatrician office said that some doctor’s offices are allowed by the ACA law to still take a smaller copay after a certain time or on specific days such as: after 5 during the week, weekend or holidays. I called insurance to find out what this was about because I was not happy with the pediatrician’s office practice of charging extra. The agent on the phone said they that the claim was submitted with a code they added for after hours. So apparently, the time of day that I work penalizes me for when I can take my kids to the doctor? Where is a law to help us with that?