Sold me an out of stock item?

I ordered some back to school Stationery items online. The most important item, which happened to be on promotion was an A3 drawing board. I paid by credit card and received my confirmation mail. A few days later I received a call to say the item was not in stock. I cannot understand this as the item is still advertised on the website (7 - 10 days delivery), it also says that the item is in stock at multiple Makro branches. I was offered a refund, but now I have lost my discount and I have to wait for my money to be returned. Firstly, how is it possible to advertise and take payment for an item you do not have? Secondly, your website says you have branches with stock? It’s not only about the loss of the discount, but also the inconvenience of having to wait for the refund and now I have to source another board, but I’m returning to work and have extremely limited time.