What extra charges? There are no charges associated with emailing documents to your Kindle. I don't believe there is anyway you can change the domain name of the email address (the part to the right of the @). You might be able to change it to be firstname.lastname@example.org but not email@example.com.
The purpose of being able to change the email address is simply to make it easier for you to remember. You do realize that this is not a true email address in that you can send & receive emails but simply a way for you to send documents to your Kindle, right?