SubjectInvoice, due yesterday
Hi [their name], Quick one: my invoice for [amount] was due yesterday. I've attached it again in case it got buried. Thanks, and thanks again for the work. [your name]
Three emails for an overdue invoice, friendly to firm, with your details filled in and the day to send each. Nothing you type leaves your device.
SubjectInvoice, due yesterday
Hi [their name], Quick one: my invoice for [amount] was due yesterday. I've attached it again in case it got buried. Thanks, and thanks again for the work. [your name]
SubjectInvoice, now 14 days overdue
Hi [their name], My invoice for [amount] is now two weeks overdue. It's attached again. When can I expect payment? Thanks, [your name]
SubjectInvoice, final reminder
Hi [their name], My invoice for [amount] is now 30 days overdue. Please pay it by [a date]. If I haven't received it by then, I'll pause any further work until it's settled. [your name]
Most late payers forgot, so the first email assumes they did. The second asks one question, because a question gets an answer. The third names a date and only says what you’ll really do. If none of them work, pick up the phone.
The longer version: how to chase an unpaid invoice. No invoice yet? The free invoice generator makes one.

Flexi can draft each one for its day, for you to send.