Preorder, Back In Stock ‑ STOQ

Exactly what i wanted! You can set the "add the cart" button to become "Preorder" button when the inventory becomes zero. There's also option to notify customer when it's back in stock. The design fits the theme perfectly. Customer service was quick and effective! and it's much more affordable than the alternative. There's also disclaimer/reminder at the checkout page to remind customer that there's a preorder item in the order so the customer is aware of the wait(you will need to edit this in the shopify language)