After an intensive ten second google search, here is what the USPS recommends:

“The Postal Service does not prohibit sending cash in the mail and reminds customers about the options they have. It is advisable to send money via a personal or certified check or money order, as these are traceable and a stop payment can be put on a check if it doesn’t arrive at its destination. If a person does decide to mail cash, it can be insured. USPS also offers Registered Mail – the Postal Service's most secure mail service.”