Just to say this still appears to be an issue - Tweeted Hertz about it and they just said to try phoning the local reservations centre, but it sounds like they'll only be able to cancel/rebook too. Given the decent cashback on the booking I guess I'll just show my card when collecting and request retrocredit - what an annoying system that it doesn't let you add it after booking, especially when the rates displayed after logging in to the #1 account are often worse than for non-members!