There is a charge if you use 3G, instead of wifi to send documents via email to your kindle.
Quote:
Whispernet Delivery Options
You can change document delivery options for your Kindle devices or set the maximum charge for a single personal document sent wirelessly over Whispernet to your Kindle with 3G support. Personal documents exceeding the maximum charge will not incur a fee and will only be available for delivery via Wi-Fi or USB transfer
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You find that under settings and scroll down. Under the text I just quote there is the option to set the maximum you can be charged, I have had that set to 0.00 for years so in case you send something and you don't have wifi, it will not try to send it with 3G and just wait until you are in wifi range again.