That tends to get confusing. My understanding is that if you have a 3g Kindle, you should use the @free.kindle.com address in order to get the free transfer. If you have wifi only, @kindle.com is all you need and is free.
"In general, send personal documents to your "name"@free.kindle.com address to wirelessly transfer personal documents to your Kindle over Wi-Fi as well as to the e-mail address associated with your Amazon.com account at no charge.
If you are not able to connect your Kindle via Wi-Fi, send your documents to your "name"@kindle.com address. The files will be sent to your Kindle over Wi-Fi if available. If Wi-Fi is not available, the files will be sent via 3G for a small fee."
I'm not sure what qualifies as a personal document. If I buy a book at a store other than Amazon and send it to my Kindle, is that considered a personal document?
If so, I've done that a number of times without charge on my wifi K3 using the @kindle.com email address.