Simply replace the unfilled areas such as "" and "[date]" with your information.
We are writing regarding the amount owing on your account. The amount of [$] is now considerably past due.
If this issue cannot be resolved soon we may be forced to hand your account over to a collections agency, which could negatively impact your credit score. We need to hear back from you by [date] for this further action to be avoided.
Please contact us at [number] at your earliest convenience.