Not my fault you oversold, replace my bag!

I bought 3 bags from foschini during the black Friday sale, I've received the 2 but yesterday I received a email that apparently 1 has sold out and now I have to be refunded for it. I sent an email and called asking if the bag can be replaced, I got no reply on my email and on the call I was told it wasn't possible from their end. The frustration is why wait 10 days? And it has happened before and it really is frustrating that they oversell products so often!!! I need a replacement bag not a refund!!!!