If a check is returned a second time, your customer may be having financial difficulties, (or he may simply be out of town on an extended trip). In either case the letter should have a sense of immediacy about it without damaging the relationship.
1234, Main Street
Boston, MA 02123
<Recipient Address Goes Here>
As stated in our <date of first letter> letter, we re-deposited check number <state check number> for <$amount> after it had been returned for insufficient funds. It was returned again due to lack of available funds. I'm sure you want to remedy this situation as much as we do. Accordingly, please send us a certified check, cashier's check or money order in the amount of <$amount> immediately.
If this presents a problem, please call <person to contact and phone number> to make arrangements to pay your bill. Thank you.
encl: <List of enclosed items goes here>