In Estate of Crossman (1964) 231 Cal. App. 2d 370, the language of the option to purchase read in pertinent part: " 'Any notice that either party hereto desires or shall be required to give to the other hereunder shall be given in writing, either delivered personally or sent by prepaid registered mail.' " The Court of Appeal held that because the provision for the use of registered mail extended to any notice between the parties, this was a permissive provision rather than a mandatory one, and notice of exercise of the option was effective when posted by regular mail. (Id. at p. 373.) "This broad inclusion of all notices, regardless of importance, implies mere suggestion of a permissive method of communication"; accordingly, "registered mail was not a prescribed requirement or an absolute condition, but rather a mere suggestion." (Ibid.)