Kindle's Personal Documents Cloud
. No USB needed. Just email or use Send To Kindle
K3, K4, Kindle Touch and Paperwhite support this. Also the Kindle Fire, Kindle iApp and Kindle Android app. K4PC doesn't.
It doesn't support epub though. Calibre will be needed to convert to Mobi.
I almost never plug in my Kindles to my PC. I use the Personal documents Cloud extensively. You will need WiFi access.