One thing that bugs me and it's happened a couple of times recently is this:

I order online and everything is in stock except when it arrives there's one thing missing. It comes later with an extra postage charge. I've queried this before and had the postage returned but it is a faff.

If something is stated as being in stock when you order but then goes out of stock surely it's not too difficult to hold the extra postage on this item?
This was particularly infuriating when the item was already a replacement for something else that was out of stock!