Postcard / Proxy Cipher Vietnam

Encrypted text reads: According to the current mailing standards of the United States Postal Service International Mail Manual, sending this postcard is a violation of United States Postal Service regulations prohibiting “invisible ink, codes, ciphers, symbols or other types of secret correspondence, and shorthand notes” mailed from the United States to Vietnam. This message is encrypted using a simple substitution cipher shifting each character three letters forward in the alphabet. Also known as a Caesar Cipher, this code was Julius Caesar’s method for protecting military correspondence. In a traditional fashion, the final ciphertext is written in fixed groups omitting punctuation and spaces. In the event of a seizure, return, or penalty from this action, the return addressee will be held responsible.