Postal Act Article 26

Railroads, motor vehicles, ships and aircrafts that engage in forwarding are required to assist in the carriage of mail items and postal personnel handling the mail items.Chunghwa Post and the forwarding agents may negotiate the carriage charge for the transport of mail items and postal personnel mentioned in the preceding paragraph.
中文原文 郵政法第26條
凡以運送為業之鐵路、長途汽車、船舶、航空器,均有輔助載運郵件及其處理人員之責。
前項載運費用,得由中華郵政公司與運送業者商訂之。

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