You may have noticed a "Pre-Order button" appear on some product pages, as you can see on the picture below:
instead of the regular "Add to Cart" button
If the pre-order button appears, it means that the size you selected in this specific design is currently out of stock but available for pre-order. It is in production in Peru and delivery will occur in April 2017.
The point of pre-ordering a pair is to receive it as soon as possible since, once received from Peru, some sizes will go out of stock again very fast. The payment is processed when the pre-order is placed.
If you have any question regarding pre-orders, don't hesitate to reach out at firstname.lastname@example.org :)