Pre-orders & Stock

How do pre-orders work? When is payment taken for pre-orders? Can I pay later for a pre-order? What happens if I order a pre-order item with an in-stock item? Can I cancel a pre-order? Can I change a pre-order? Why has the release date changed for my pre-order? How do I know if an item is in stock? What does lead time mean on a product page? Can I be notified when an item is back in stock or released? I cannot find a specific model. Will it become available? Can I suggest a model for you to stock? Can I swap a delayed or out-of-stock item for another model?