To purchase the extras, go to subscription and payment, then click on "Buy New" and the list below will be displayed:

After selecting the extra you want, click confirm  

Then, you will be directed to the checkout page. You can increase the quantity and duration according to your needs. Then validate the terms and conditions and follow the payment process by clicking on "Proceed to Checkout".

N.B: only those with a valid plan will be able to purchase the extras.