Threatening to turn the account over to a collection agency is a last resort. There is a serious problem and you have probably lost this person as a customer. Because there is a chance that this might be caused by a misunderstanding, the customer should be given one last chance to pay.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
We have not heard from you since our letter of <date of last letter> informing you that check number <check number> for <$amount> was returned by your bank a second time. At that time we requested that you send us a certified check, cashier's check or money order or call us to make other arrangements.
If we do not receive payment within ten days of the date of this letter, we will be forced to turn this account over to a collection agency. In order to avoid damage to your credit rating and associated difficulties please remit payment immediately. If you have any questions, you may call <person to contact>.
encl: <List of enclosed items goes here>