Firstly, keep the tone of the letter warm and friendly. Non-payment may simply be an oversight and you don’t want to offend the customer, you just want to get paid.
123 Lane, Area 456,
City, State, Pin Code
May 18, 2012
(Mention Recipient’s Address Here)
Would you please check your records? As of (date checked) we had not yet received your payment. You currently have an outstanding balance of ($amount). This amount was outstanding on (date payment was due) (please refer to invoice number (invoice number)).
If you have already made the payment, please ignore this notice. If you have simply overlooked it, please take a moment now to mail us a check. Thank you for your prompt attention to this matter.
encl: (List of enclosed items goes here)