A late payment
When: An invoice is a week or more past due.
Subject: A quick note about invoice [number]
Hi [first name],
I hope your week is going well. A quick note: invoice [number] for [amount], from [date], still shows as unpaid on my side. It may simply have slipped, which happens to all of us.
Here is the link to pay it: [payment link]. If you have already paid, just let me know and I will look on my end.
Thank you, [your name]
Tip: Send it on the same day each week, such as Friday, so it is a routine and never a confrontation.