Swiggy instamart - My order had missing item

Swiggy cheated me on pretext of a freebie which it popped as a reward to me and the same then temped me to place an instamart order. So the order was delivered but there was no freebie in the same and when I reached out to customer support they told the item was out of stock. This seemed very convenient and I feel I have been cheated by Swiggy on pretext of a freebie. While the reward was given it was not mentioned that item is subjected to availability and the same was added to cart on applying the offer (I mean no e-commerce website adds item to cart and let you checkout if the item is running out of stock then why this?) but the same was not delivered.

If it was not for the freebie I would have picked fewer items from a local shop at walking distance only. This feels like a trap being run by Swiggy to gain more customers or to meet some order targets.