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I read the manual for my cars radio. It has a max file size limit of like 256 songs or so per folder. But it can also accept 256 folders.
So if your cars is anything like mine you can probably play your songs just by splitting them up into more folders.
Good news is that there are alternative ways to download these books from Amazon for backup purposes. It’s not as straightforward but it’s doable.
That said I will be refusing to buy from any storefront that doesn’t offer a way to download my books. Even adobe digital editions is a viable alternative.