Supplementary answer: You are right. Books could have been mailed, and a small amount of imports could be tax-free. However, China has a censorship system for foreign publications. Therefore, the import of some books is not only a matter of quantity, even if it is a book, the contents inside are suspected to be flawed and cannot be mailed (to the mainland).

It takes time to judge whether it is justified by the customs. In order to avoid delaying their promised mailing time, express delivery companies generally do not accept the mailing of books and magazines at some critical times, which is understandable.

Have your documents been sent to the mainland? Because of the censorship system, the courier company is worried that the delayed delivery of mail will affect its business, so it tries not to accept books and periodicals. It can only be sent by post office (more expensive).