Invoice & Payment Tracker is a ready-made tab for the invoices you send. Every invoice has a number, a client, an amount, an issue date, a due date and a status - draft, sent, paid or overdue - and one click on Mark paid closes it. Across the top you see what is outstanding, what is overdue and what you were paid this month.
It also adds a task called Overdue reminders. Run it from the Tasks tab whenever you like: it finds every overdue invoice and writes a short, polite reminder into that invoice's notes, ready for you to copy into an email. It never sends anything. The task is added switched off - turn it on from the Tasks tab when you are ready.
A few sample invoices are included so you can see how it works; the Remove sample data button clears them when you are done. Everything stays on this computer.