Asking Extra Amount After Delivering The Product

I have placed an order for 3 items in pcjewellers.com website.2 orders have been delivered and they cancelled the 3rd order. While placing the order for two items it has shown 6250 inr to pay for each order. I have paid the amount and got this both order delivered. For 3rd item I have used a coupons of 26k to buy the item. After I got two items delivered I have contacted the pc jewellers for third item. At that point of time they cancelled third item in order and they were asking me to pay extra 26k for the two items which was delivered earlier. They are saying I have done fraud and threatening me. I have done nothing. While purchasing the items I have paid the amount what was shown on the website and made payment. Now they cancelled the 3rd order which I have applied coupoun of 26k and now they are saying that coupoun is applied for two items which got delivered and asking me to pay extra 26k now. While purchasing the order they cheated me by showing less price for items. What should I do now?