On the first notice, keep the tone of the letter cordial. Non-payment may simply be an oversight and you don't want to offend the customer, you just want to get paid.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes 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 due on <date payment was due> (please refer to invoice number <invoice number>).
If you have already sent us your payment, please disregard 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>