Amai PreOrder Manager + Notify

As other reviewers have said, I'm also using this primarily for backorders instead of true pre-orders, but it helps immensely. Being able to cap the number of orders taken is exactly what we need to manage our stock and keep selling when items are almost back in stock but not quite there yet.

When I've had questions or issues with the app the support has been great, I recommend it highly if you need this functionality in your shop.