(a) On each installment in default for more than 15 days, a creditor may collect a delinquency charge that does not exceed the lesser of an amount equal to five percent of the installment or $20.
(b) Only one delinquency charge may be collected on an installment, regardless of the period for which the installment remains in default.
(c) The charge or collection of a delinquency charge does not affect the right of a creditor to accelerate the debt under this subchapter.