The absolute safest way to send something in the mail is to send the mail through certified mail.
Certified mail costs around $6.50 for the certified part and ensures the letter or package that is certified gets to the intended recipient.
Also never send cash through the mail as it can easily be stolen and once you lose that cash it's gone forever.
If you want to or need to send money through the mail send the money through a money order or personal check.
I've had cash sent to me as gifts before and it made it okay but there's been some times that the cash was stolen but thankfully it was not that much money.
Also use security envelopes to send any checks, money orders etc that someone cannot see through.