Domino's Pizza — charging extra taxes

Respected u bought pizza from muzzaffarnagar. Firstly asking for the bill they said that they will give it with the delivery. Ok.. At the time of delivery i checked the amount and found 50rs less from the refunded money. They said that by mistake it happened. Checking the bill later on i found that the tax for the different amount of artical is same that is 18%. Means that one artical is of 69 and second one is of 119 for both the articals they are charging same price i. E.17.16rs. Now my querry is how is it possible. Plz help me out