Theda Care Medical Center- Waupaca

We do owe money to the medical center and have been paying $100.00 a month. They wanted to raise it and we said no so they said they would send it to collections. We received the bill from collection agency yesterday. So now it’s on our credit even though we pay on it every month. Now we actually have to pay $108.00 a month to collections agency. So for $8 they sent us to collections. How can they do that if we pay something on it every month?