Dear ___ (name):

Thank you for your generous order received ___ (time period) which you
requested to have shipped immediately and charge to your account.

However, you have probably overlooked the fact that your
for ____ (period of time) your account has been past due. Our policy has
always been to suspend credit until all past monies are paid. Therefore, we
request that you remit the amount owed from the previous statements.

After receipt of your check, we will promptly ship your order.

Yours truly,