That plugin allows you to sell products even if the stock has reach zero.
Your customers will know that the product is not in stock yet and would be able to buy it like it was.
Pre-order change the display and few interfaces but it do not interact with the product price or the payment procedure.

Features

• Set-up pre-order stock for your products
• When the product quantity reach zero, "add to cart" becomes "pre-order"
• Customer can pre-order for the value you have configured