You can buy additional mailboxes as follows:
Log in to your Spaceship account using your credentials
Once there, go to Spacemail Manager
When you’re in Spacemail Manager, find the relevant subscription and click on Get mailboxes now
Set the number of mailboxes you want to add. You’ll also be able to see the price for them there
Click on Proceed to checkout and pay for your order. Once done, you’ll be able to create your new mailboxes.
NOTE: Additional mailboxes cannot be purchased for trial subscriptions. If you have a free trial Spacemail, you’ll need to upgrade to a paid version first to be able to purchase additional mailboxes.