I did that once in China. 1500 Rmb to my GF in Anshan. She said, she never received an envelope.

Try to mask money with something. Pack of Chinese tea, and stick money in it. Open tea nicely, put cash in it, and close the box again. Tea must look the same as before you open it.

Or anything else. Baby dolls, toys, and so on….

Much safer if you use Western Union or bank transfer if the amount isn't large. Or try doing it in HK. What I do to send money back is to open a bank account here and send the card by registered mail. Call and tell your friend the PIN over the phone. That works only if your country accepts Union Pay. 

I've tried sending a novelty hongbao to a young family member. The woman at the postoffice gave me the cash from the hongbao back to me along with the return money. China Post does not allow cash. Maybe try hiding it inside a book, but you risk loosing all, why not wire it and just loose the fees

Use a courier like UPS by mail it will take 3 to 5 weeks to get to the US if it get's there.

Western Union, UPS or FEdEx if you don't want to lose it.

Wouldn't recommend it, but if I had to I would at least use EMS so I could track it to its destination.

Yeah… this sounds like a really BAD idea.