It's rare for me to see a promo code for Audiobooks.com for more than the usual 1 credit, but I just spotted one
https://www.lovereading.co.uk/2-free-audiobooks. Typically these are for new members only, but it doesn't state that here. I'm in the middle of a month on my membership or I'd try it out myself.
Enjoy!
skinmaan
Your audiobook bloodhound