The remaining balance email reminder is scheduled in advance. If a payment is made after it being scheduled the system can sometimes still send the message. Just a case of signals crossed, especially when the payment was made recently or close to the reminder email's sending date. If the email doesn't apply to your situation you can ignore it. No worries.
You can always check the balance of your booking aka see if there is a remaining balance via the Online Check-in Portal or the Secure Payment System. Both systems tap directly into our booking system aka your booking so you have access to the same info as our crew. Power to you! 💪