(a) The policy may contain provisions under which the company will accept advance payments of premiums; but in no event may the company undertake to accept deposits that would exceed the maximum amount required to pay all future premiums that will become due under the policy, including any options contained in the policy. The contract may permit the insured to withdraw excess deposits in cash, but any provisions that would cause a forfeiture of principal or exact a surrender charge are prohibited. The contract must state the interest rate used to discount the future premiums and must provide for disposition of any unused premiums on surrender of the contract or death of the insured.(b) This section is not applicable to single premium policies.