Daxto's autopilot automatically sends a payment reminder 3, 7 and 11 days after the due date of an invoice. This is more formal than the invoice reminder, but gives the customer sufficient time to still pay the invoice.
If you hadn't contacted them yet, now is the perfect time. You can then blame Daxto for sending the payment reminder. Computers are unstoppable, right? You then show all understanding towards the customer and discuss together how the payment can still be made as quickly as possible. In this way, you are the 'good cop' and your Daxto assistant the 'bad cop'. But your assistant doesn't mind that.
If the payment reminder doesn't help, then you can consider a collection agency and eventually mark the invoice as bad. But before that happens you can mark the invoice as doubtful.