Section 22A. Notwithstanding any contrary provision of law relating to collective purchasing, but subject, however, to all other laws regulating public purchases and competitive bidding, the commonwealth and one or more of its cities, towns, districts, counties, or authorities, or two or more cities, towns, districts, counties, or authorities, 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 said purchasing agent; provided that the political subdivision shall accept sole responsibility for any payment due the vendor for its share of such purchase.