How to enable pre-ordering for your store products?

You need to select the checkbox "Allow customers to purchase this product when it's out of stock" for each variant of such products in admin panel of your Shopify store. It can be made on product's variant edit page, as it is shown on this screenshot:

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Or it can be made on Inventory page as it is shown on screenshot here:

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• Page:
  What is Split Payment & Deposit?
• Page:
  How does the app affect standard templates created in the store?
• Page:
  How can I hide/show some invoice elements depending on order status?
• Page:
  What happens to the product inventory when the app is used?
• Page:
  How are taxes applied to recipients when any Split Payment method is used?