The service charge shall include all charges incident to investigating and making the retail installment account. No fee, expense, delinquency, collection or other charge whatsoever shall be taken, received, reserved or contracted by the seller or holder of a retail installment account except as provided in this section. A seller may, however, in an agreement which is signed by the buyer and of which a copy is given or furnished to the buyer provide for the payment of attorney's fees and costs in conformity with § 4345 of this title.