2. Go to Private Email menu on your left, and then click Manage next to the needed subscription:
3. Scroll down to the section with your current mailboxes. Click on a Buy Mailbox option from the dropdown menu and you will be redirected to the payment page:
NOTE: The maximum number of additional mailboxes that can be ordered at once is 999. There is no limit to the total number of additional mailboxes you can have with one subscription.
The prices of additional mailboxes can be found here.