So far, sending files to your Kindle
cost a flat fee -- one dime per document for conversion and download over Whispernet. Looks like that honeymoon is over, as Amazon's announced that as of May 4th, the Personal Document Service will be a variable fee of $0.15 per megabyte, rounding up. It's still free of charge if you transfer the documents over via USB, and sending them to "name"@free.kindle.com will return converted files to your email address gratis. If you're trying to be frugal, we might suggest combining all those pending transfers into one fat PDF and sending it off sometime this weekend.