The 'Send to Kindle' is a misnomer. It's Send to Amazon. Then it has to come back to your own network and Kindle, so your account settings on Amazon and on the Kindle and model and firmware affect how it works.
Also if you buy from Amazon and make 'too many' returns they will close your account and the Website library will be gone.
Put all the ebooks on your own computer(s) (all of them is good), remove DRM and manage with Calibre.
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