Below is the procedure for adding mailboxes to your Google Workspace subscription.
Go into your control panel and locate your Google subscription. Enter product management by clicking on "active".
Select "Upgrade/Downgrade Options".
Type the new configuration, i.e. the number of total boxes you want.
An invoice is then issued which covers the difference in cost of the new boxes until the next expiration of the existing boxes. Once the invoice is paid, the new box will be available.
Subsequently, by going to the Google control panel, you can create new mailboxes.
