Disappointed

There was a special online of 200ml juices. I ordered 10 x 6packs for R160. I was called a couple of days later that they do not have any stock available, therefore they will cancel my order. I told them that I do not want to loose out on the offer and would rather wait until they replenish their stock. They said they have to cancel the order because they have to close the sale, they can't keep it open. I said they can close it and give me a voucher to buy later, they were not keen. I found this extremely unfair and unreasonable. Why do they continue with sales when they do not have stock at hand. I am very much disappointed.