That is one of the reasons to shop direct where you can and source from those vendors that maintain a live up to the minute inventory 24/7/365. This is why I do business with Sprue Brothers:
"Inventory Questions
We have been
receiving a lot of inquiries lately regarding whether we actually have
something in stock as our website indicates. This is likely due to the
practices of other online hobby stores who have a different definition
of what "In Stock" means. In case you are not familiar with our
business practices, we use a real time inventory system that shows
(accurately and live/in real time) what is available for sale and how
many. It automatically and instantly updates the available quantities
after each order is successfully placed.
Also, our definition of “In Stock” means that
the item is actually in our warehouse (not at our
distributors’/suppliers’ warehouse) and is ready to immediately ship to
you. Thus if you order from us, you will reliably and quickly receive
what you order. On a side note, we are also human, so on rare occasion
something could be in error, however our error rate in 2009 to date has
well been less than 0.1%."
There are a lot of online retailers out there, and many don't actually stock (that is have possession of the items they are selling) on hand to ship immediately when you place your order. Nothing worse that having a package arrive from the vendor with a note of what's not included and will be shipped at a later date. Double that with they're gonna charge you additionally for the cost of the subsequent shipment too.
As a manufacturer, I make it a point to update my website when a particular item is currently not available. It may be that a vendor I depend upon hasn't serviced my needs, in turn I can't meet yours. Of course these situations don't happen often, but they do happen once in a while. If you did place an order and for some reason I was not able to fill it completely, then I take the lumps and send it as soon as I can with no additional cost to you. It wasn't your fault. It was the way I was taught, treat others as I would be treated.