When purchasing your Impress.ly site, it is possible to pay yearly or monthly. 

1. Login to Impress.ly using your account
2. Open the Impress.ly site you want to purchase
3. Press the Publish tab in the horizontal menu.
4. In this menu, you can find the different 'Buy' buttons
5. Press the button to purchase the yearly plan of your choice or click on the blue link below this button to pay monthly.