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For category music CDs what is the placeholders for artist?

{$Artist}

Are there any special placeholders for MP3 downloads?

There is a special built-in template for mp3 files. Use [asa mp3]ASIN[/asa]

The plugin does not show the price for Kindle ebooks correctly.

Unfortunately the Amazon webservice does not support Kindle prices at the moment. See https://forums.aws.amazon.com/thread.jspa?messageID=208072

I get the PHP-Error: Warning: domdocument::domdocument() expects at least 1 parameter, 0 given in \wp-path\plugins\amazonsimpleadmin\lib\Zend\Service\Amazon.php on line 129

There are two very similarly named PHP extensions, dom and dom_xml - the dom extension is built into php5 and loading the dom_xml extension (designed for php4) will override the default extension. I suspect that loading php_domxml.dll (on Windows) in your php.ini is the cause of this problem. Try to disable it and see if that helps.