Each person insured by a mutual assessment property and casualty insurer shall be issued a contract prescribed by the insurer, that shall be uniform among members of the respective classes of insurance written by the insurer. Each member shall agree to pay his pro rata share of all losses or damages sustained, expenses of operation of the insurer, and the maintenance of an adequate surplus to policyholders as determined by the board of directors. Periodic assessments may be collected as advance premiums or post assessments or by both methods. The amount of assessments shall be established by the directors of the insurer.