Audible has an offer that can earn you $20 in Amazon credit. From now until March 3, members who finish 3 titles that are at least 3 hours long will earn a $20 Amazon credit. That’s it. You’re already enrolled if you received the offer directly by email. All you have to do is just start listening.

You can see the promotion here. Looks like you need to have received an email in order to qualify.

Offer Terms

• Offer available until March 3, 2020 at 11:59 p.m. PT.
• Offer will only be available to Audible members who are in a trial or membership as of 1/31/20.
• Eligible Audible members with a linked Amazon account who listen to three entire titles that are at least three hours in length between January 1, 2020 and March 3, 2020, will receive a promotional code for a $20 USD Amazon promotional credit.
• Promotional codes will be delivered via email on or around March 17, 2020.
• You must be opted in to receive promotional emails in order to receive the reward email.
• You must redeem the promotional code within 30 days of receipt by clicking the link in the email.
• Unless an Amazon Gift Card is the stated benefit of the promotion, promotional codes or credits (including those placed directly in accounts) may not be redeemed for Amazon Gift Cards.
• Offer is limited to one per customer and account, may not be combined with other offers, is non-transferable and may not be resold.