This is to acknowledge our receipt of your invoice.
We are unable to process this invoice due to the fact that you have omitted our purchase order number.
Please forward same to us and we will make our remittance promptly.
Contained herein is the [identify product], purchased on [date], which is still under warranty.
I have enclosed a copy of my bill of sale for this purchase.
The problem that I am experiencing is as follows:
[specify] I would be most appreciative if you would either make the necessary repairs or send me a replacement. Thank you for your prompt attention.