A bank is authorized to act as custodian or fiduciary, and may receive reasonable compensation for so acting, under any written trust instrument or custodial agreement in connection with a tax-advantaged savings plan authorized under the Internal Revenue Code or chapter 30, title 63, Idaho Code, if the funds of such trust or funds subject to the custodial agreement are invested only in savings accounts or deposits in such bank or in obligations or securities issued by such bank. All funds held in such custodial or fiduciary capacity by any such bank may be commingled for appropriate purposes of investment, but individual records shall be kept by the custodian for each participant and shall show in proper detail all transactions engaged in under the authority of this section.