Buy 3 for 2 Special

Upon receiving my order from Woolworths I discovered that there were 2 items missing from the package. Both shoes. I then phoned Woolworths to notify them and was told that the one item was out of stock and they money will be refunded into my account and for the other pair they will re-deliver. I then phoned again today to follow up and was told that the other item will not be refunded as it was a free item and that it is out of stock and I won't get it. My argument is that when I bought the special was buy 3 for 2 and I managed to add all my items in my cart and checked out successfully. May I please get assistance with this because clearly I did not save anything. And the item still shows on my order slip.