A public procurement unit may either participate in, sponsor, conduct or administer a cooperative purchasing agreement for the procurement of any supplies, services or construction with one or more public procurement units or external procurement activities in accordance with an agreement entered into between the participants. The Department of General Services is authorized to enter into cooperative purchasing contracts solely for the use of local public procurement units or State-affiliated entities. The department shall enter into cooperative purchasing contracts for the use of local public procurement units or State-affiliated entities if the number of contractors under a previously existing contract for the same supply or service is reduced to a single contractor or reduced by more than 50% of the number existing on September 30, 2003, and shall award such contracts pursuant to section 517 (relating to multiple awards) using an invitation for bids. Nothing in this section shall prohibit a local public procurement unit or State-affiliated entity from participating in or procuring from other cooperative purchasing agreements awarded by the department. Cooperative purchasing may include, but is not limited to, joint or multiparty contracts between public procurement units and open-ended purchasing agency contracts which are made available to local public procurement units.