Here is one that someone from this site was nice enough to share with me a few years ago. I have had to use it a few times and had great results after sending. Hope it helps.
[Enter Date Here]
Customers City State And Zip
This letter will serve as formal notice to you that you are in default of your obligation to repay the
sum of . The service was provided to you on (List Service Dates Here) (Reference Invoice #’s
This amount has been overdue since (Put Date Here) and you have failed to repay the same
despite repeated requests for payment by (Company Name Here) LLC
Unless payment of the above amount is received by us in full within ten (15) days of the date of
this letter, we will have no alternative but to exercise whatever rights and remedies we have
under the law to enforce such payment, including but not limited to institution of legal proceedings
against you to recover the above amount, together with accrued interest and legal expenses. Let
it be known that a police report is on file for theft of services for the above matter.
Kindly govern yourself accordingly.