can a doctor charge an extra?

yes can do that other places businesses do this as well

A doctor has the right to demand full payment for his/her services at the time those services are rendered, as does any other professional, like a plumber or a shoe-repair shop. If you cannot afford to pay up front, there are only a few professionals nowadays who will extend credit to you to allow you to pay over time. Those few who do almost always will charge you interest on your outstanding balance. Most plumbers will simply refuse to do business with you if you can't afford to pay, and tell you to put it on your credit card.

Your doctor is doing you a favor by allowing you to pay your outstanding balance over time. If he/she is not charging you interest on your balance, then they have every right to charge you a service fee every month, which helps to offset the extra time and expense it takes his/her staff to print your monthly bill and send it to you, and then process the check you send in each month.

If your doctor is charging both interest and a service fee, I would question his/her staff about it. That would be unusual.

If you are paying your bill fully each month, then it sounds a bit fishy. If you are charging it, makes sense.