Dear…
RE: Billing Error
I have recently received a bill from your company for __($amount)__ along with a letter that states my account may be turned over to a collection agency if not paid within __(#)__ days. Needless to say, I am very upset about this because, in fact, I have no outstanding debt with your company at all.
Please provide me with an immediate explanation for this insult. As a customer whose been in good standing with your company for __(length of time)__, I think I deserve to be treated better than this.
Yours truly,