Add an item to the bottom of the queue

⇒        Click on the last item in the queue, then on an item in the inventory list. Then click the Add button, or double click on the item in the inventory list.

⇒        To add additional units, keep clicking the Add button or double-clicking on the item in the inventory list.

Large orders
Pressing CTRL+SHIFT when clicking on the Add button will add as many of that item as possible. This also works if you press CTRL+SHIFT and double click on the item.