Regulations Governing the Handling of Mail Article 42

Mail items shall be transmitted by surface transport or air transport depending on the postage paid.After an airmail item is posted for delivery, should it fail to be transmitted by air due to reasons that are not Chunghwa Post’s fault, Chunghwa Post may send the mail item via another fast method and the airmail postage paid shall not be refunded.
中文原文 郵件處理規則第42條
郵件按其所付資費,分別由水陸郵路、航空郵路運送之。
航空郵件收寄後,因不可歸責於中華郵政公司之事由,致郵件未能交由航空郵路運送時,得改由其他較為迅速之郵路運送,所付航空資費不予退還。

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