Del. Code tit. 6 § 4337
Subject to the other provisions of this subchapter a retail seller or the holder of a retail installment account may charge and collect a service charge computed on the outstanding unpaid indebtedness in a buyer's retail installment account and may calculate such service charge in the manner and at such daily, weekly, monthly, annual or other periodic percentage rate or rates as the agreement governing retail installment account provides; provided, however, that if the service charge as so computed is less than $1 for any month, the holder may charge $1 as a service charge for such month. If the applicable periodic percentage rate under the agreement governing a retail installment account is other than daily, the service charge may be calculated on an amount not in excess of the average of outstanding unpaid indebtedness for the applicable billing period, determined by dividing the total of the amounts of outstanding unpaid indebtedness for each day in the applicable billing period by the number of days in the billing period. If the applicable periodic percentage rate under the agreement governing the retail installment account is monthly, a billing period shall be deemed to be a month or monthly if the last day of each billing period is on the same day of each month or does not vary by more than 4 days therefrom.
6 Del. C. § 4337