You can purchase units online and set minimum balance remainder for your account from here.
Go to Admin > Billing and Reports > Purchase Units.
You can purchase units online by entering the amount of units to purchase and then click on the "Proceed to Billing" button.
Once clicked, you will be presented with the credit card processing window.
Simply fill out all the required field and click on "Pay Now" to place the order.
Low Balance Reminder
You can additionally set a minimum balance reminder.
This will send an email reminding the users selected that the balance of the account is below a set threshold.