When a client's balance drops to a certain level, the platform can automatically send them an email notification. You set the threshold yourself — once the balance hits that amount, the client gets notified so they can top up in time.
Go to Profiles → Clients.
In the Actions column, click the edit icon next to the client:

In the window that opens, find the Notification limit field and enter the balance amount at which the notification should be sent. The amount is in the client's currency:

Click Save.
The client will receive an email notification as soon as their balance reaches the amount you set.