Subject: Friendly reminder — invoice [number]

Hi [customer name],

I’m following up on invoice [number] for [work/project], due on [date]. The remaining balance is [amount]. I’ve included [a copy / the invoice link] so you have the details handy.

If payment has already been sent, please share the date or reference so I can match it to the invoice. Otherwise, could you let me know when I can expect payment?

Please let me know if there is a question about the invoice.

Thank you,
[your name]
[business name]
[contact information]