便中

qǐng biàn zhōng gào zhī

请便中告知。

Please notify me at your convenience.

wǒ tì nǐ shāo lái yī shuāng xié qǐng biàn zhōng lái qǔ

我替你捎来一双鞋，请便中来取。

I've brought you a pair of shoes. Please come for them whenever it's convenient.

biàn zhōng qǐng gào zhī yú tā

便中请告知于她。

Please inform her of it at your convenience.