Buyer Beware: Several years ago, metrofax was a great service adn would allow custom cover sheets. This is no longer true. They were taken over by j2 global which moved them to their inferior efax platform.
Other things you should consider before going with metrofax. They automatically charge your credit card for another year if you are on the annual plan, and notify you after the fact. They also will not port any number they assign to another service, thus you either remain with them or have the expense of changing your fax number with all your clients. Both of these are spelled out in their contract, but are reasons to avoid this service.