Just update to version 0.3.1 with some functions you were requesting.
A new field "Reply-To" can be used to force a specific Reply-To email.
If you use some plugin (such a contact forms) that does fill the headers with a Reply-To email, you can set this field to
'headers' (without quotes)
and the plugin will try to extract the email from the headers and use it if available.
Another function is a global configuration for WPMU, of if you don't want the blog admin to see your amazon credentials.
You can configure some constants in the wp-config file, such as amazon key, and force or hide some items.
I will document this feature later on.
See plugin FAQ to see what constants are to be used.
Thanks to all for the various feedbacks, please continue to suggest new features and report bugs.