You can find examples of invoice letters at Nolo.com or Writinghelp-Central.com. The type of invoice letter samples you may need depends on the type of letter written. An example of an invoice letter is the combination of an invoice for services or product written in letter form, or the letter could serve as a demand for payment after a product was purchased or a service rendered.
When writing an invoice letter that is summarizing and requesting payment for a product or services, the writer may want to start out by stating that the letter is in regards to products or services recently purchased or rendered. From there, outline the details of the billing, including the units purchased or time used as part of the service.
If you are writing an invoice letter for a demand for payment, meaning the customer has yet to make payment on previous invoices or bills, you can start out by summarizing the history of the account. Try to stick to the main facts of the dispute, and outline what your goals are regarding a possible resolution, such as a settlement for partial payment. Finish your letter by advising the customer that if payment is not received soon, legal action may follow.