In reply to JuneBob:
Thanks for the replies.
We're not really in the most convenient location. He'll be in Saas Fee when I arrive in Geneva, then I drive to Saas Fee and meet him, and I go on randonnee tour to Zermatt while he drives to Zermatt. So, if we need to go to an office it's quite an extensive round trip.
I've emailed all the rental agencies and they all pressed reply template 13b or something. I'm following it up, but they're being quite stubborn.