The term "additional fee" refers to an extra charge or cost that is imposed on top of what was originally agreed upon, quoted or estimated. It can be applied in various situations such as when a service provider charges more than what was initially stated, when there are additional expenses during the completion of a project, or when taxes and surcharges apply to an already existing fee. The phrase is commonly used in financial transactions, travel bookings, legal agreements, and other contracts where extra costs may arise.