You can also email files to your Kindle. First, go into your Amazon account and "Manage Your Kindle." Somewhere in there is a spot where you can set the maximum amount you'll allow to be charged per book sent. Set that to zero. Second, if your kindle email is, for example,
samdeathwalker@kindle.com, stop using that one. Use
samdeathwalker@free.kindle.com instead. Now it will never, ever go over 3G, only wi-fi. Then you can email stuff to your Kindle and it'll download it the next time you connect to your wifi.