I think you also need to photocopy your spouse's passport main photo page and include your own passport and photocopy.
We sent a copy of the lease rather than just a letter, it is more formal, together with photos of the apartment to show that there was sufficient room for us to live there.
In addition we sent copies of correspondence between us over the internet, chats and emails together with evidence of the financial support I was providing my fiancé/spouse while she was in phili. These were copies of the transfers of funds.
