CA Gov't Code Section 13995.49

The commission may by written contract accept a voluntary assessment from any person in a travel and tourism related business who is not an assessed business. The contract shall apply solely to the person in question and not to any other person in a travel and tourism related business that is not an assessed business. The contract shall provide that the voluntary assessment be proportionately equivalent to the assessment that would be levied if the person were an assessed business under this chapter, shall permit that business to vote on any referendum conducted under this chapter as if that person were an assessed business, and shall have a term concurrent with the effective period of any referendum on which the person votes. Individual voluntary assessments under this section shall be enforceable only under the terms of the respective contracts to which they pertain. This section shall not be construed to preclude donations to, or cooperative marketing activities of any kind with, the commission on the part of any person.