As you mentioned, the proof is in the GC. If it ever got to a legal standoff in court, you do not have to prove that the CA responded. The CA has to prove they responded via a copy of the response or validation and they would likely have to prove you received it (their own GC).
Faxing has been proven to legally be OK, but the problem I have is the tranmission sheet can easily be fabricated or doctored. I can easily drum up an exact and duplicate copy using MS Word, for example. Then the only back up would be phone records, but your phone carrier may not be to open in sending you those.
Do you work Monday-Saturday? I live in the DC metro area and here, at least (I'm very sure not everywhere), I have access to a post office that is open until midnight each night and is also open on Saturdays and Sundays. I have full access to all of their services during those times. Maybe there's a post office near you like that.
I remember a USPS salesman came by the office once. He offered to have packages sent overnight from our office via USPS. At the time we didn't need the service, but I thought how would they track the packages if I mailed from here? Maybe there's a way to have everything addressed prior to mailing it that the mailman would pick up and process himself/herself? Don't know.
