On a revolving charge account, the extender of credit, issuer of a seller credit card honored by the extender of credit, or their assignee, may charge and collect a credit service charge in each billing period at a rate
(1) not in excess of one and one-half percent per month computed on (a) the average daily balance of the account or (b) the balance of the account on the first day of each billing period without regard to transactions affecting the account during the billing period; provided, however, that a minimum credit service charge not in excess of fifty cents per month may be charged and collected; and provided further that no credit service charge shall be charged unless the bill is mailed no later than ten days (Saturdays, Sundays and legal holidays excluded) after the billing date stated on the bill; or
(2) any other method of computation which would not yield a greater credit service charge than subsection (1) of this section.