Sample email asking about a charge

Use this as a starting point and keep it calm and short.

Subject: Question about invoice #[number] from [date]
Hello [Company name],
Your technician serviced my [heating/cooling] system at [address] on
[date]. Before the work began, I approved a written estimate of
$[approved amount] for [brief description].
The invoice I received is $[billed amount]. It includes [item], which I
did not approve before the work was done.
Please remove this charge or explain it in writing within 14 days.
I have attached the signed estimate and the invoice.
Thank you,
[Your name]
[Phone number]