Price mistake and claim they're out of stock

When ordering a bottle of perfume of R399 I wanted to add something which costs R1 to get free delivery. I searched for pens. I came across 50 pens for R5 after a 98% discount. I ordered a few of these as it was really cheap and a day later I got a mail stating that this product was out of stock. This is not the first time that this has happened to me. It happened to a couple of friends as well.