Yes, its the best way to make sure there is no miscommunication.
We use a Free service from WeTransfer.com. About three hours of 16 bit/44.1 kHz WAV is about 2GB so keep your files under that and sign up at WeTransfer (took us about 1 minute), and we're in business.
EDIT: Now you don't even have to sign up!
Yes! We are in service to you, not the other way around.
Yes!
As most in this industry, billing is done with every hour that is produced for you. So for example if you asked us to produce an audio book for you, we would charge for every hour of that audio book. That includes the narration and/or technical production. Administration is free.
I'm told it's not necessary, so until someone tells me otherwise the answer is no.