Del. Code tit. 25, § 5311

Except for an optional service fee for actual services rendered, such as a pool fee or tennis court fee, a landlord shall not charge to a tenant any nonrefundable fee as a condition for occupancy of the rental unit. Nothing in this section shall prevent the tenant from electing, subject to the landlord’s acceptance, to purchase an optional surety bond instead of or in combination with a security deposit.