In order to purchase additional mailboxes for your Hosted Exchange please follow the steps provided below:
1. Login to https://my.ozhosting.com
2. Please click "Buy Additional Resources" on the Store portion
3. Select which subscription you would like to get an additional mailbox for and click the "Next" button
4. Under the "Additional Mailboxes:: Professional and Basic" portion, change the current value to desired value and click the "Next" button (e.g. changing the value from 2 to 3 will create 1 additional mailbox)
5. Select desired payment method and click the "Place Order" button (please take note that on this page you can verify the subscription ID, amount to be paid, and the items being purchased)
Once the order has been placed allow for a few mins to have the mailbox provisioned.