What does EMS International Express mean by sending me a short message for customs declaration?

That is, your goods are too valuable to declare.

Announcement No.43 (20 10) of the General Administration of Customs of the People's Republic of China stipulates: 1. If an individual posts imported articles, the customs shall collect import duties according to law, but the amount of import duties applied for is less than RMB 50 yuan (including 50 yuan), which shall be exempted. Two, personal items sent from or sent to Hong Kong, Macao and Taiwan, each time the limit is 800 yuan RMB; Items sent from or to other countries and regions are limited to RMB 1000 yuan each time. Business declaration is required for those exceeding RMB 1000.