Customers are emailed when their payment fails (Card payments) so they are made aware at the point it fails.

If you're using an integrated smart entry solution you can set the unit to overlock automatically when the payments fail. This will turn the unit red on the map.

You can manually place an overlock on the unit to indicate an issue with payment for sites not integrated with smart entry.

Using the invoices tab in Stora you can filter for open and draft invoices. Open invoices are those that are still outstanding or where payment has failed.

Draft invoices generate when the previous invoice/invoices have continuously failed. They can be charged by clicking into the invoice in Stripe. If you have a customer that is a continuous non payer it is useful to also check for draft invoices.