Each Kindle email address has two variations: @kindle.com and @free.kindle.com. The @free.kindle.com will only transfer documents over Wi-Fi, so that the user is not charged. If you use the @kindle.com email address, it may instead be transfered over 3G at a cost to the user. They can configure their settings to disallow this, but you should be careful not to send documents to this address unexpectedly.