Usually plugins will include wp-config.php or wp-load.php in order to gain access to core WordPress functions, but there are much better ways to do this.

It's best if you tie your processing functions (the ones that need but don't have access to core functions) into an action hook, such as "init" or "admin_init".

Please consult the Plugins API reference for more information: http://codex.wordpress.org/Plugin_API

When you've corrected your code, reply to this email with the updated code attached, or provide a link to the new code.

1) Upgrades will not be done by your site. I know what you mean to say is that the code they're calling from your site will be maintained by you, and that's fine, but by saying upgrades, people think the plugin files will be upgrade, and that's not the case ;)

2) Along that same line, you need to be more clear about how the plugin runs code on your site. Just so people know exactly where their data goes etc.