If the contract does not specify how a bill must be sent, then any delivery mechanism, including email, is valid. So you cannot hold out for hardcopy specifically. (Note: in the future, if this matters to you, write it into the contract.) However you have a right to hold out for legible billl with whatever support or documentation is necessary, also legible. So you can require them to resend anything which you can't read or otherwise rely on as a bill; but if they resend adequate and legible documentation, it doesn't matter if it's via email, fax, U.S. mail, hand deliery, fed ex, etc. All that matters is that it is legible and adquate in form, with whatever information or documentation that is necessary.