We booked though Avis (under the deals tab at the top that says "you can now take your Avis car to the Snow".
Put in our dates etc. Given a final price $200 and booked. Read all the T&Cs (not many would). See you can't drive over snow.
Call up to confirm. Oh yes. When you arrive at the airport and they think you are going to the snow you will be charged an extra $150.(which means if they didn't ask and you have an accident up there you would have no cover). 2 great ways to find out about their "deal".
Call centre confirmed there is no indication of this charge on the mobile site and no way it can be added unless you call up or pay when you arrive to pick up.
Website booking apparently suggests calling, but doesn't say there will be an extra cost. No indication that you won't be covered on the mobile site at all. I rang head office and got no response. Poor business behaviour.
..Went and booked with Apex, chains only extra $25 to hire for the duration.
Thanks for the update tunzafun. Glad you didnt let Avis ruin your holidays.