I found this just now from another post just down the list:
If you have a Wifi+3G, and you send it to the @free.kindle.com, it will only download once you're in range of Wifi. If you're out and about, you won't get the delivery.
If you have the WIfi+3G and you send it to the @kindle.com addy, it will download via Wifi if you're in range, for free, or via the cellular connection if you're away from wifi, in which case it charges you.
Since I have a wifi only K3, I can only d/l when in range of a hotspot so I'll never get charged and don't need to worry about setting my max fee to $0.00.
I should have clarified that I d/l my non-Amazon purchases to my PC and via calibre, send them via email to my K3 using the @kindle.com address. I only sideload if the filesize is too big to send via email.
I haven't tried Project Gutenberg yet. Will have to check them out.