Swiggy - Cancellation fee wrongly charged

I ordered Item A + B on swiggy and prepaid for the order. The delivery boy called me to inform that half of my order (Item A) is not available. He offered me to replace missing items of order with Item B only. If I would have wanted Item B, I would have ordered that. They should not display item on the menu if those are not available. I asked him to cancel the order in such case. He told me to go ahead and cancel the order. He never informed me that I will be charged with half of my refund amount in name of the cancellation fee.
When I contacted customer support, they refuse to call me and informed me that they won't refund my money completely. They charged me for services they can't even provide. It's like either I am forced to accept whatever order they will deliver or else I have to be ready to lose money.