Amazon just revealed a few changes to their Kindle supported documents as well as modifications to their whispernet fee structure. You can now send both DOCX and RTF files wirelessly to your Kindle and they’ll be converted automatically for viewing. The fee structure for personal documents (books that you purchase remain free to download) has changed from being .10 per document. Starting May 4 you will be charged .15 per megabyte (rounded up to the next whole megabyte) for any personal documents that you send to your Kindle email address for conversion and upload.
Free document conversion is still supported by sending attachments to “firstname.lastname@example.org”.
Starting May 4, in addition to the existing list of supported file types (DOC, HTML, JPEG, GIF, PNG, BMP, TXT, AZW, MOBI, PRC), you can send RTF files to your Kindle email address for convenient wireless delivery. In addition to the existing experimental support of PDF, you can also send DOCX files for conversion. Some complex PDF and DOCX files might not format correctly on your Kindle.
We have also modified the fee associated with sending personal documents wirelessly to your Kindle. This fee is now based on the size of your file. The fee for Personal Document Service (via Whispernet) is 15 cents per megabyte rounded up to the next whole megabyte.