This article delineates the difference between NOT PAID and PAYMENT DUE invoices.
NOT PAID INVOICES
An invoice is marked as Not Paid when the payment has not been received. No further attempts will be made on this invoice. An invoice is also marked as Not Paid if a direct debit transaction fails.
PAYMENT DUE INVOICES
An invoice is marked as Payment Due when a charge has been attempted but has failed. Chargebee will continue to retry the charge based on the dunning settings. You can also choose to collect the payment immediately by clicking on the Collect Now button. An invoice is also marked as Payment Due when payment collection via direct debit is in progress.