I was supposed to write an apology letter for delay in payment of membership fee to a business group, but i did not know where to put the membership no.
My teacher said that the membership no should be written in place of reference no. like this.
                                                                                           .......street.......
                                                                                        .......Bldg, Office no.....
                                                                                        Dubai, UAE

                                                                                        membership no: 200888

(my company does not use reference nos in letters)

My boss said that the membership no. goes above the salutation.

membership no:200888

Dear Sir,

what i would like to know is:-
Q1) Who is right my teacher or my boss?
Q2) Should membership be written with a capital 'M'?
Q3) Where do I mention the other company's reference no.?
Q4) The letter is to be sent from Dubai (a city) to Dubai (Within United Arab Emirates, which is a country) should I mention UAE in sender's and recipient's address in the letter?
Q6) Does the same rule apply to Invoice no.?
Q7) Where do I write the membership no (or Invoice no) in an email?

Thanks in advance.