Takealot not willing to assist customer

I placed an order late yesterday afternoon, the order has not been shipped as yet. I made a mistake on a size regarding one item I ordered. The order is paid but I only want to cancel the one item from the order and need a refund. Phone Takealot only to be advise that I cannot cancel the item as the warehouse is picking and when I asked to cancel the whole order I was advise I cannot do so ( had an issue with takealot last year as well and manage to cancel complete order and get a complete refund?? ) This is very bad, I spent a lot of money and they cannot accommodate their customers , the order has not been shipped yet I do not want to wait for the order and do a return, it is time consuming. The order has not been shipped yet so you can cancel the item. Not happy