If these are real I would seriously put tracking on them. It protects you and the winner. If they don't arrive at winners home without tracking you will lose your credits and the winner will have no way to know if they're shipped. It's worth the couple dollars to send. I speak from experience. =)
Just thought I would throw in my 2 cents, white letter envelopes are put through machines that press the Postmark on the envelope, this often crushes items in the envelope (I learned the hard way, lol), also it is always better to have delivery confirmation, it cost 85 cents and is worth the money spent, it protects you from someone claiming they never got the item or from people with sticky fingers at the Post Office, both of which have happened to me.
Putting diamonds in the hands of the usps without tracking scares me... You can print delivery confirmation for free on usps site with postage purchase, thats my suggestion. Also write fragile on the envelope at least then they will hand sort it.