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.
Access to updates included for 4 years
A tax rate (between 17% and 27% based on your country) will be added if your address is in the European Union and that you don't provide a valid VAT number or if your address is in France as per the European Union law.