Yes. You may fold a money, a check or a money order and mail it. Checks can be folded and still be good. Folding it does not affect its negotiability.
When sending checks through the mail, you should "hide" them. You can try to make your letter look like a plain-old boring letter instead of something containing a large juicy check. Do this by folding a thick piece of paper around the check.
Get more advice on what can you do to keep your money from ending up in the wrong hands by reading TheBalance.com's Tips on How to Mail a Check Safely.
And here are the basic steps on how to send a cashier's check, shared by Sapling.com.