Deontological ethics is a type of morality that claims that the morality of an action is based on whether that action is right or wrong based on the set of regulations that society operates under. Therefore, it emphasizes the "duties" that people have within a society.
In this situation, a deontologist would most likely argue that Agnes' situation is moral because she is fulfilling her obligation of paying her taxes. While she pays at a discounted rate, the deontologist would argue that different situations imply different duties.