A payor may require a health care provider to extend the payor’s medicaid rates, or some percentage above the payor’s medicaid rates, that govern a health benefit program administered by a public purchaser to a commercial plan or line of business offered by a payor that is not administered by a public purchaser only if the health care provider has expressly agreed in writing to the extension. For the purposes of this section, “administered by a public purchaser” does not include commercial coverage offered through the Washington health benefit exchange. Nothing in this section prohibits a payor from utilizing medicaid rates, or some percentage above medicaid rates, as a base when negotiating payment rates with a health care provider.