[Title (if required)]
[Department (if required)]
[Organization (if required)]
Dear [First/Last name],
I hope this letter finds you well.
I am writing to bring to your notice an error in the September invoice [#] addressed to me.
The invoice says that I owe you $[#], while in fact the actual outstanding amount according to my calculations is $[#], which is less than a quarter of what your invoice mentions.
I would also like you to know that this is not the first time my invoice is showing an erroneous outstanding amount.
If you check your records, you will see that I was sent erroneous invoices on not one but four previous occasions since I signed up for your service in March 2009.
I am sure you will appreciate my patience in writing to the company directly on each occasion instead of going the legal route.
I would now like you to know that any future erroneous invoices will force me to cancel your service and/or result in a lawsuit.
I must also add that your service is the best I have experienced so far. It is unfortunate that my perception of your company is clouded by a billing error.
Please do let me know if you have any questions.
I look forward to seeing this billing error problem resolved amicably and permanently.
Please sign and return the attached photocopy of this letter for my records.
Thank you for your time.
[Account details (if required)]