Got this after a recent car rental from Alpha. They were already $60 cheaper than anyone 'at' the airport (they do a shuttle to.from their base which is 10 minutes from Melbourne Airport), so an even better price with this.
They're in a few different places, see http://www.alphacarhire.com.au/ for info & a quote.
The email said:
As promised please find a $20 off car hire voucher attached as a thank you for your comments. Feel free to pass this offer onto family, friends or colleagues who may also benefit.
So I am!!
I can't access the voucher!!!