You can use this format:
[LETTER HEAD OF EMPLOYER]
Date:
Re: Our Letter/Invoice dated _________ no._________ (if any)
Sub: Reminder for payment of the above referred invoice
Dear Sir/Ma'am,
With reference to the above, kindly note that, the due date for the payment of the same was _____________. However, it is observed from our Accounts that, the same has not been cleared yet.
Hence, kindly consider the same and arrange to clear the same as early as possible/ within a period of ___ days/ upto_________date______
Thank you.
Yours faithfully,
-sign-
Name
Designation