Irrevocable — Any changes (amendment) or cancellation of the LC (except it is expired) is done by the applicant through the issuing bank. It must be authenticated and approved by the beneficiary.
Use 'Yours sincerely,' when writing to a named person. Use 'Yours faithfully,' when using the 'Dear Sir/Madam' greeting. Use the semi-formal 'Best regards,' or 'Kind regards,' if you wish the tone to be slightly less formal.