You can also set a maximum cost-per-day. People can email you a document that would cost $329874 to convert, but $329874 is greater than what you've set, it will be rejected.
You get a special 'secret' email for your kindle. You aren't supposed to share it with people, so it's relatively secure. You can also change it at any time if someone does manage to get it.
I do something similar with my Sony Reader; save to instapaper, instapaper to epub, epub to ereader. Obviously the book has to be hard-wired through USB for this to work, but then it also doesn't cost me anything.
It runs the epub through calibre convert, because the sony reader has issues sometimes with the instapaper epub file - the conversion fixes it (something to do with chapters over a certain length).
Through the evening, I instapaper save anything interesting to read, then just run this script each day before I leave the house.
This is the particularly rare-groovy batch code if anyone's interested.
If you instead input username@free.kindle.com, it sends it to your email so you can download it and put it on your Kindle.