If you have access to a PC, I would use Sigil with the EpubCheck plugin to check your epubs before sending them. It will correct some issues such as missing doctypes which even EpubCheck no longer complains about but Amazon seems to have issues with.
Edit: This is based on my exerience with both downloaded and created by calibre epubs and has only been tested on ~16 epubs. YMMV. Caveat emptor.
Last edited by DNSB; 07-02-2022 at 03:22 PM.
Reason: Adding limited experience disclaimer
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