The upcoming payment due notification only works on current charges but excludes late fee charges. The "amount due" merge field does not include carrying balance or past due amount from previous months. For example, as of September 20th, an account has a past due amount of $100, a current charge of $500 (charged on September 1st and due September 30th) and a late fee of $10 (charged on September 1st and due October 1st). In the notification body, the amount due only shows $500. This does not make sense. The amount due should be the total account balance of $610 on September 20th, the notification send date.