Search for any audiobook and click on its .
: You usually must have 0 or 1 credits left in your account.
If you are an active member, you can often purchase bundles of directly through Amazon's website.
To buy Audible credits on Amazon, you typically need to visit an audiobook's product page, as there isn't a single "storefront" page specifically for buying standalone credits on Amazon.com . How to Buy Extra Credits on Amazon.com