Bigbasket.com — Big Basket is cheating customer in the name of order modification related charges

I ordered 3 kg mangoes on Bigbasket on June 15. I received an order modification mail from Big basket on June 16 morning ~1.30 hours before order delivery. It was mentioned that the mango weight is 3.36kg instead of 3kg and for that, they are charging ~51 Rs for the extra .36 kg which would have been found during the packaging. After receiving the order I was told they updated me that the mangoes are only 2 boxes i.e. 2 kgs, not 3 boxes and I will be refunded accordingly. I received a refund for that 1 box based on the exact 1kg. However, I have the following 2 questions:
1. If they do not have a sufficient quantity of the item it should have been known by the time of order packaging itself means when the order modification mail goes to the customer that should highlight this point as well since it's just 1 hour before the delivery.

2. I got a refund based on the exact 1 kg while I was charged extra for .36 based on 3.36 kg. Ideally, I should be charged based on 2.xx kg of order and refund also should be in the form of 1.xx kg.

So, even the refund amount is quite minimal I believe this is cheating a fellow customer either deliberately as part of a scam or some issue in their processing style. But, crores of customers are cheating in this manner and Big basket should correct that.