Section 22A. Notwithstanding any general or special law relating to collective purchasing, but subject to all other laws regulating public purchases and competitive bidding, the commonwealth and 1 or more of its cities, towns, districts, counties, authorities or commonwealth or Horace Mann charter schools, or 2 or more cities, towns, districts, counties, authorities or commonwealth or Horace Mann charter schools, hereinafter called political subdivisions, may make purchases of materials, supplies, equipment or services through the state purchasing agent subject to such rules, regulations and procedures as may be established from time to time by the purchasing agent; provided, however, that the political subdivision shall accept sole responsibility for any payment due the vendor for its share of such purchase.