If you've run out of your monthly allowance, here are the most effective ways to stock up:
While you can't strictly "buy" Audible credits as a standalone product on the standard Amazon retail site, you can purchase them through your linked account on the Audible site or by finding an audiobook on Amazon and selecting the "3 extra credits" bundle option on the product page. How to Get More Credits buy audible credits on amazon
If you are a paid member for at least 30 days and have 0 or 1 credit left, you can buy a 3-pack of credits. This usually costs around $35.88 $30.00 to $35.00 depending on current promotions, bringing the per-book cost down significantly. If you've run out of your monthly allowance,
Switching to an annual plan like Audible Premium Plus Annual gives you all 12 or 24 credits upfront, which lowers the average cost per credit to about $9.50. Important Restrictions Switching to an annual plan like Audible Premium
A popular community hack is buying the Kindle ebook first (often for $0.99–$3.99). This often unlocks a heavily discounted "Add Audible Narration" price, sometimes as low as $1.99, which is cheaper than using a credit.
LitRPG Books - Discounted audible credits for kindle users - Facebook