Section 5C. Any vendor under any assistance program administered by the department shall submit to the department, within six months of the last day of the month in which such service was rendered, a bill for the same; except that, in cases in which services have been rendered in accordance with the provisions of section twenty-four A of chapter one hundred and seventeen, said bill shall be submitted to the department within six months after the date on which the request for assistance has been approved. The department shall verify the accuracy of bills submitted under this section through the application of statistical sampling methods. For the purposes of this chapter a vendor shall be any person or institution providing services in connection with any assistance program administered by the department. All vouchers submitted by a vendor shall be signed under the penalties of perjury.