Section 22B. Notwithstanding any contrary provision of law relating to collective purchasing, but subject, however, to all other laws regulating public purchases and competitive bidding, any two or more political subdivisions, as defined in section twenty-two A, may join together for the purpose of obtaining and accepting competitive bids on similar items of materials, supplies, equipment or services which they intend to purchase, provided that each political subdivision shall accept sole responsibility for any payment due the vendor for its share of such purchase. No political subdivision which serves as a purchasing agent under the provisions of this section shall be liable solely by reason of its actions as such agent for any claim based upon a breach of warranty or defects in the design, manufacture or installation of the materials, supplies or equipment purchased pursuant to this section.