Yep - have Send to Kindle and I have Calibre also set up for sharing by email to my various Kindles/apps.
Oh - hold the phone! I think I have something. I was able to use my "Manage Your Kindle" page to deliver personal documents to my PW despite the fact that I have "Whispernet Delivery over 3G" set to disabled in my "Personal Document Settings" page.
From this, I take it that the only option that is charged is the email to Kindle functionality. The other self-service options - "Cloud" or "Archive" on the Kindle itself and the "Manage Your Kindle" page are both free services via 3G.