'Pro Rata Fee' is ONE TIME fee attached once to price of the server at the time you purchase it. Pro Rata Fee is a Fee which covers the days from renting the server until the end of the same month in which you made the purchase. With this one-time fee paid you server will be active until the end of the next month based on the order date.
You make a purchase on 15.12.2017 / Here you have already paid Pro Rata Fee which will cover the days from 15.12.2017 to 31.12.2017 . Your server expiry date: 31.01.2018 / Here you have already paid at the purchase the fee for the server it self for a whole one month.