BTW, to be precise, the foregoing analysis assumes we’re talking about an ordinary check, where the recipient is identified as the person to be paid (or his order), e.g., John Doe or ABC Corp. If you sent through the mail a check payable to the order of bearer or “cash,” such a check can be stolen. I assume, almost trivially, that this isn’t the scenario we’re discussing.
Registered mail is the ticket if you’re being cautious.
You’ve got something resembling a chain of custody with it.
But if it’s actually made out to a particular individual… send it first class isn’t a terrible idea.
If you are the State of Pennsylvania you stick it in regular mail. A guy I know hit the lottery for a few hundred thousand dollars and got a check regular mail.
Just put it in regular mail, but be sure to put on the back of the check: For deposit only.