Undignified business

I asked for a small size. They delivered a medium size.My father was going to bring this dress with him to Canada for me. As soon as I got the dress I called myntra and very politely and nicely I told them that you had delivered the wrong size. He said the exchange 

period is only within a week. I told him that my father did not know how to check the women dress sizes. I requested if they could please exchange it or refund me. They bluntly denied. The window of exchange was within a week. I told him that how does he expects my father to travel and come back within a week if there is anything wrong with the product. They were heartless and had no decorum to stand up to their mistake