The United States Postal Service does not prohibit you from sending cash in the mail, as long as it's not for an illegal purpose. But, it's still not a good idea, especially when sending large amounts of money. There are safer alternatives to sending cash, such as USPS postal money orders.
Protecting Your Cash
When sending cash in the mail, you can minimize the risk by insuring it for the amount you are sending. If the cash goes missing, you can file a claim with your local post office. You'll have the burden of proving the exact amount of cash mailed before you can be reimbursed. The USPS recommends that you ask your local post office what it will accept as valid proof of the amount of cash before you mail it. Once your claim is filed and accepted by USPS, you will be reimbursed only up to the amount of insurance you purchased. If you send cash by registered mail, you can insure the value for up to $25,000.
A safer way to send cash in the mail is by purchasing a USPS money order. Available at any post office, it can be purchased for any amount up to $1,000, per money order. The value of the money order can be reimbursed if it is lost or stolen; it can also be cashed at any local post office and at many banks. Money orders can be purchased for less than $2 with cash, debit card or traveler's check. Personal checks, gift certificates or gift cards are also safer alternatives to mailing cash.