US Post Office warning: Don’t send this in the mail during the holidays

There are all sorts of things you're not supposed to send through the mail and most are pretty common sense: fireworks, handguns, drugs. Other things are less obvious or require special handling, such as live animals, old-fashioned thermometers that contain mercury, nail polish and perfume.

But there's one common thing the U.S. Postal Inspection Service recommends against sending via the mail, especially during the holiday season: cash.

USPIS's warning on sending money through the mail is straightforward: "Customers should never send cash in the mail."

It's not illegal to send cash in the mail, of course, but if it's stolen or lost in transit, there's nothing you can do. Instead, it's better to use a secure electronic transfer method like a mobile payment app or wire transfer. If mail is the only option, USPS recommends using a money order.

"Your money order receipt will help you track your payment and show proof of value in case the money order gets lost, stolen, or damaged," USPS said.

What about mailing a check? That is an option but it presents challenges as well. If you do mail a check, never make it out to cash or leave it blank and always wrap it in paper before placing it in the envelope to make it harder for crooks to see what's inside.

You should also use a gel pen to write out your check. Experts said the ink in a gel pen makes it harder for thieves to "wash" the check and change the payee names and dollar amounts. It's also recommended to drop checks (or any financially sensitive mail) at the box inside the post office instead of leaving it in a mailbox with a flag up – that can send a signal for thieves to drop by as well.