It would be good to be notified if credits are about to expire. Ideally the notification gives users enough time that something can be done about it, but if at least it’s sent a week or so in advance it would be already helpful.
In this notification, I would also make it clear that one can’t use credits from the current month to book a lesson in the following month. This is by the way not obvious on the FAQ either. I lost credit because I booked a lesson before the month was over, but the lesson was booked for after the renewal. Chatterbug gave no indication the credit was being deducted from the upcoming quota.