How to do preorders

You can do pre-orders with the Waitlist feature by simply setting the inventory count for the product to 0 in Shopify:

Then when you get the product in stock. Update the inventory appropriately in Shopify. This automatically triggers the "back-in-stock" messages for anyone on the waitlist:

Shopify does not allow for pre-authorizations. But you can charge the customer's credit card for the purchase if you select this option in Shopify: