Upon written demand by the purchaser made at the time of the notice of dispute as provided in section 70-1304, the supplier shall, each month, deposit the disputed portion of the monthly payment as specified by the purchaser, but not to exceed twenty percent of the total amount billed, into an escrow account, in a bank located within the county of the principal place of business of the supplier. Such deposits shall be made from the date of the demand until settlement of the dispute as provided by sections 70-1301 to 70-1329. The escrow funds shall be invested, to the extent possible, in securities of the United States Government and the balance in other insured interest-bearing accounts of the bank.