Swiggy — unjust cancellation fee

There was a bug in the app, and the coupon which was shown to be applied made the order total to be 156. I clearly saw it 3-4 times, and tried making payment too but when i tried to make payment it shooted up to 236, so i cancelled the payment and thought it is because of the payment method. So i came back and tried to make the payment again. Again the total was 156, this time i chose ' By Cash' method.
The order total was 156, but as soon as the order got placed it became 236. And i called to cancel within the minute thinking it was a bug in their app.
But they simply halted me, and charged me cancellation fee for order.