Mr Price Home - buy 3 pay for 2 items

I ordered 3 items on promotion buy 3 pay for 2. When i received my order I only got 2 items .when I enquire regarding my other item,the online consultant said that the item was out of stock and that why I forfeited the third item. I requested a substitute item of same value but,consultant said no I can only get a R50 voucher as compensation for the third item. I send an email to them and are waiting for them ,because I want a substitute item of same value. I purchase this order at buy 3 pay for 2 promotion. I was so disappointed.