Amazon doesn't sell them via their app to avoid the app-store-tax. Instead they'll redirect you to their website in safari. You can borrow KindleUnlimited titl

Amazon doesn't sell them via their app to avoid the app-store-tax. Instead they'll redirect you to their website in safari. You can borrow KindleUnlimited titles, provided you've purchased that subscription outside of the app. Same thing with Audible, you can use existing credits within the app, but can't purchase more.