Originally Posted by KimberlyFDR
And I double-checked that many times when I first got my Kindle last year
I send a lot of documents to my Kindle every day (A LOT) and always download over wi-fi for free. Since I don't have a 3G version of either the Kindle or Kindle Fire, I can't comment for sure, but you should be able to put your document cost limit to $0 under Manage Your Kindle and that will force download to only be over free wi-fi.
You can also just disable the feature entirely. I have mine set that way so it always go to WiFi and never 3G.