Background: Most packages entering the US valued at under $800 (from any country) are exempt from import duties. This greatly expedites small imports, since there's no need for a broker to arrange payment before they're released at a port of entry (a process which generally costs several times the actual duty and adds delays).
When Trump imposed additional tariffs on China, Canada, and Mexico, he also eliminated the de minimus for those countries, requiring a lot of new work at ports of entry.