A credit union is authorized to act as custodian or fiduciary for members of the credit union 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 credit union. All funds held in such fiduciary capacity by any such credit union may be commingled for appropriate purposes of investment, but individual records shall be kept by the fiduciary for each participant and shall show in proper detail all transactions engaged in under the authority of this section.