Swiggy — cancelled my coupon after successful order without notifying me

I have ordered around 171rs of 3 items. Swiggy called me and there is not available of one item so they exchange with other and they confirmed that rs5 will be refunded to my account. Exchange item is less value. But while receiving the order they said i need to pay extra 53 rs. Since my coupon is not applied by swiggy. I called to customer care. But they said i need to pay the amount. Totally waste of money...