Poor System Protocol

I ordered 5 items before December and out of the blue, I received an email saying one item is out of stock, they have removed the item form my order and they have placed a credit on my account. Take note this was cheaper than 450, therefore i would need to buy for more to get free delivery or pay for the delivery. After complaining in writing I get told they can only cancel and not amend orders. I even managed to phone the customer care line and no where could they assist. I ended up cancelling more items as they could not swop it for a different colour/version that was in stock at a cheaper price. What is so difficult. They should have a system where telephonically can adjust or add to your order if items are out of stock and how do they allow out of stock items to be purchased.