If this is pre-interview, I would worry that just sending it snail mail would be too slow at this stage of the game. Not only does it have to get there, but someone has to physically open it, and I would imagine the volume of mail each school gets is enormous. I know I wouldn't waste the extra money to mail it Priority, as it's going to get there maybe a day earlier, but still most likely will be held up on the end of the school itself anyways.
Some schools actually prefer that you email them these sorts of materials, as it makes it easier on them. If you don't know what type of correspondence the school prefers, maybe send it both ways?? Or post up here what school(s) you're sending the letters to and maybe we could help you out with their preferred method? I emailed all of my update letters last semester, and nearly every school emailed me back within a day or two to say it'd been added to my file. I also just emailed a letter of interest post-waitlist.
