clarity07,
Going by what people have experienced on the forum (including me), if I were you, I would send my send a USPS Priority or Express (ensuring that I get an earliest Priority Date form USCIS) with a Return Receipt (so I could track when my package reached USCIS). Also, I would pay by Personal Check as against Money Order (as this would allow me to track when USCIS actually withdraws money).
Since you must have been checking earlier posts on this forum, make sure you send proper amount, sign the application in full name, and have attached all the needed documents. All that information has been covered in several stickys and previously posted messages.
Good luck to you.