A district may include along with, or as part of its regular customer billings, a request for voluntary contributions to assist qualified low-income residential customers of the district in paying their district bills. All funds received by the district in response to such requests shall be transmitted to the grantee of the department of commerce which administers federally funded energy assistance programs for the state in the district's service area or to a charitable organization within the district's service area. All such funds shall be used solely to supplement assistance to low-income residential customers of the district in paying their district bills. The grantee or charitable organization shall be responsible to determine which of the district's customers are qualified for low-income assistance and the amount of assistance to be provided to those who are qualified.