Subject: Friendly nudge — invoice [#1234] ([$AMOUNT])

Hi [CLIENT NAME],

Hope things are going well. Just a quick note that invoice [#1234] for
[$AMOUNT] was due on [DUE DATE] and I haven't seen it come through yet.
It may well have slipped through the cracks — happens to all of us.

I've re-attached it for convenience. If it's already on its way,
please ignore this!

Thanks,
[YOUR NAME]
