Then here is some more free information.
After changing the output settings, select all books in your calibre library, and use Edit Metadata.The Bulk Edit Metadata screen will come up -- check the box on the bottom labeled "Remove stored conversion settings for the selected books".
This will remove the stored value for the PDOC tag, for books you have already converted to MOBI/AZW3.
Without this, the cached value is used when you hit Send-to-device, leading to a great deal of headscratching.
You can also install the Quality Check plugin and use the option Fix ==> Fix ASIN for Kindle Fire, which embeds a false ASIN in the book and ensures cdetype=EBOK.