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