(a)service thereof shall be deemed to be effected by properly addressing, prepaying and posting a letter containing the document, provided that where a member has intimated to the company in advance that documents should be sent to him under a certificate of posting or by registered post with or without acknowledgment due and has deposited with the company a sum sufficient to defray the expenses of doing so, service of the document shall not be deemed to be effected unless it is sent in the manner intimated by the member; and