Postal Act Article 6

Except Chunghwa Post Co., Ltd and its entrusted parties, no one may, for the purpose of business, deliver the following documents except for cross-border documents sent to and from other countries:1. Postcards2. Aerogrammes and letter sheets3. Letters:(1) Sent by government bodies (organizations) at all levels, public schools, public legal persons, public institutions or military units. (2) Sent by parties other than the senders provided in the preceding item, where a single item with a weight less than 500 grams or the postage of a single item not exceeding 13 times the basic postage. With the exception of delivery notices connected with shipments, no forwarding agency may deliver mail as described in the preceding paragraph.
中文原文 郵政法第6條
除中華郵政公司及受其委託者外,任何人不得以遞送下列文件為營業。
但國內、外互寄之跨境文件,不在此限:
一、明信片。
二、郵簡。
三、信函:
(一)各級政府機關(構)、公立學校、公法人、公營事業機構或軍事單位所交寄。
(二)前目以外之人所交寄,且單件重量五百公克以下或單件資費不逾十三倍基礎郵資。
運送業者,除附送與貨物有關之通知外,不得為前項文件之遞送。

English translation for reference only; the Chinese text prevails. Not legal advice.