Exclusively for Book Depository customers. (Some users reported that there is no check, but others reported that the offer does not work.)
New members sign up for an Audible 30-day trial membership at www.audible.com.au. You'll receive 2 membership credits for $0/month during the first 30 days of your membership (the “trial period”) instead of 1 credit. Second credit may be added at any time during first 2 days of trial period.
Terms and conditions say "This special offer is exclusive to Book Depository customers" but there is no check. My wife signed up and received the second credit within 5 minutes. Hope this helps someone.