2 sentences

1. 我正在邮局发信。

   wǒ zhèng zài yóu​jú fā​xìn.

   I am mailing a letter at the post office.

   Source: Dong Chinese Dictionary

2. 发信前请留个底。

   fā​xìn qián qǐng liú gè dǐ.

   Please keep a copy for your records before sending the letter.

   Source: Dong Chinese Dictionary