Thanks - "As of 2018, US companies pay more than twice as much to mail an item from a US plant to a US customer, than does a manufacturer in China to mail an item to a US customer."

So they are paying by the Kg, bulk not the package.

Thanks to your lead I also found this: ePacket delivery is standard small packet airmail shipping from China Post but with a steep discount for being purchased online. China Post just has to worry about paying for packages to leave China. Due to international treaty, once they arrive in the destination country, it is up to that country's postal service to pay for the rest of the delivery.

And the third part:

They use China Post or Hong Kong Post to handle the delivery. This is the cheapest carrier but also the slowest. Hone Kong Post is even up to 50% less than China Post.

Compared to international express courier (such as UPS/DHL/TNT/FedEx) 3~5 days, the post transit time can last 3~5 weeks. Though more time needed, most buyers won’t expect to receive in the next day, if they choose buying direct from China.

More luckily, the seller’s location near Hong Kong. He/She may choose a freight forwarding service to bring the goods to Hong Kong and ship from there.

Besides, they send bulk, or consolidating with other items to the same destination. That small box you received was sharing the same plane with thousands of others transported to your city.