Amazon is running one of their rare promotions for free ebook credit between August 5th and August 12th, 2020. If you visit the landing page and spend $30 in total, you will get $8 in credit that will be automatically applied to your account within 4 days Amazon will notify you via email once your credit has been applied to your account. This promotion is only applicable for US customers.
Free credit for purchasing ebooks normally occurs every five months. The last time Amazon ran this promotion was in November of last year. Past deals had users spend $50 and get a $10 credit, so this recent one is the better deal.