All purchases of commodities made by any county purchasing official with county funds, except those specifically exempted by this chapter, shall be made as follows:
(1) Formal bidding shall be required in each instance in which the estimated purchase price shall equal or exceed twenty thousand dollars ($20,000);
(2) Open market purchases may be made of any commodities where the purchase price is less than twenty thousand dollars ($20,000); and
(3) No purchasing official shall parcel or split any items of commodities or estimates with the intent or purpose to change the classification or to enable the purchase to be made under a less restrictive procedure.