From what you said, it looks like she only told you the basic fee and easily forget to tell you the fee of the additional service. It is not right but it may be difficult to fight it w/o hurting your credit history. So probably you should pay it since it is a small amount, and learn a lesson:
In America, if at all possible (i.e. you can speak decent English), do not deal with Chinese in business dealing except the following:
1. Immigration lawyer. The Chinese lawyers know the tricks.
2. Chinese restaurants. No one else can cook it.
3. Chinese grocerers. You can not buy the stuff elsewhere.
4. If you have alot of money, maybe a Chinese cpa to prepare the tax for you. They know the tricks.
Don't flame, I learned too many lessons from my compatriots.