Please provide name, mailing address, email address, and all other useful contact information.
Can you provide a short chronological list (i.e.. regular bill on such and such date, asked once a month for 5 months, etc.)
(i.e. half paid now, half paid on a date or 1 payment a month for 4 months, etc.)
A week from the letter? A few weeks? A month? Several months?
Meaning, if they don't pay by such and such date, you will stop services if you are presently helping them even though they owe you, you will take them to small claims court, civil court, reach out to their subsidiary companies/a related contact, report them to someone (i.e. professional association or other), or other?