Every morning at 10:00, find issued invoices that are unpaid and exactly 14 or 30 days past their due date.
At 14 days, email the customer a polite, personal reminder with the invoice number, amount, and due date, and a link to pay it in the app. Assume it's an oversight and keep the tone friendly.
At 30 days, don't email the customer again. Open a support ticket for the invoice's location, assigned to [your billing administrator], with the customer, invoice number, amount, and the reminders already sent.
Don't change, void, or reissue invoices. Write in the customer's language.