(a) A retail installment contract may provide for:
(1) any amount of time price differential permitted under Section 353.104, 353.105, or 353.106; or
(2) any rate of time price differential not exceeding a yield permitted under Section 353.104, 353.105, or 353.106.
(b) The time price differential may be computed using the:
(1) precomputed earnings method;
(2) scheduled installment earnings method; or
(3) true daily earnings method.