Subject: Thank you — and one small ask

Hi [CLIENT NAME],

Payment received — thank you, and thanks for being great to work with
on [PROJECT].

One small ask: most of my work comes by word of mouth. If you know one
person who could use [SERVICE], a quick intro or forward of this email
would mean a lot. And if you'd write one sentence I can quote on my
site, I'd be grateful.

Until next time,
[YOUR NAME]
