Unfortunately we do not have examples or templates for Demand Letter here at the AAP program. A google search revealed DHS used to (it may still be active, but it is not on their supplement at Acq.Gov) have an appedix to their supplement that included Demand Letter Templates. 

Another approach you can take is to post this to one of the DAU Communities of Practice; your peers in the workforce may be able to help you out.

Finally, contact the payment office, I am sure they have many examples they could share with specific information redacted.